Former Navy myself morbidDon. My ship spent about a year in the yards for a "modernization refit". Trust me, they didn't change much!

I've worked in two different shipyards for a couple of years,. Assuming the huge scale of the ships and the huge scale of the manpower they can throw at them is about in equal ratio to what we can do nowadays I'm going to call the times in the books (A week per component I believe?) very generous.

The most successful, quickest, and most efficiently run large project, the life extension refit of a large passenger ferry, took over a year. And that one ran like a well oiled machine and wound up under the projected date (which is pretty much unheard of).

On the other hand there are similar projects that have run into structural problems, snags, or eff ups and wound up ballooning up to five years from one and a half.

On a gaming front, our Rogue Trader actually had his Flagship spend two years in Drydock at the Lathes after a particularly nasty battle. Which lead to four quite interesting adventures while they hired free traders to ferry them around, hob-nobed with the Sentillan nobility and generally spent money and met people. It even led to the king-making on a mining world, taking out a chaos cult and the Rogue Trader's wedding.

So, while one can easily handwave a year or so repair/refit time, "while you're waiting" can also make for a great game starter.

Depending on where you're stuck, I can imagine, as the above material reflects, that the downtime for a ship's repair/upkeep/modifications could actually make for some interesting experiences. You might spend a month, or more, in the warp, with only you and your crew to chat with, but if you're stuck above Scintilla, for instance, you might actually take the time to get to know some movers and shakers, some important folks who can help your dynasty. As an aside, you might also have a nice scenario where these meetings HAVE to be nice, on the part of the party, as they can't just "run for it", the same way they might if their ship was ready. One of my RT characters is a complete party hound, and this might be a nice excuse to actually stay on Scintilla, and hob knob at the noble's party, rather than fly off, and steal more dead xenos leftovers. As an aside, when a real big, important Rogue Trader, like one you've made, whose loud about it, or some of those in Lure/Abyss return to Imp space, do the nobles just come up with a BS reason to have a party, and hope said big wig comes to it, so they might be seen to associate with one another (even if they've never really spoken)? On the one hand, Rogue Traders seem to like to stay beyond His borders, where all their power works, but sometimes it's nice to come back to "civilized" space, and mingle with other seemingly important people. Not sure how Aedan would be there, to sweep up all the ladies, if they didn't just plan around him, what with the vagaries of warp travel, and communications, and such.

However, you could also consider this - http://wh40k.lexicanum.com/wiki/Lord_Daros

Basically, a Lunar-class cruiser constructed in orbit of Feral World using only AdMech factory-ship and raw materials harvested by the locals in only 11 terran years. My guess, it wouldn't be far fetched to presume the actual build times for most ships - from escorts to cruisers is actually only several years if we take the shipyards in orbit of hiveworld.

However, the size of the ships is also a feat to be taken into account - for example even a lowly escort vessel of Imperial Navy from WH40K universe may be in fact larger than typical battleship from, say, Star Wars universe (for examle Firestorm-class frigate hal overall lenght of about 1800 meters, while Imperator-class Star Destroyer (the iconic Galactic Empire's warship from the movies) is only 1600 meters). Imperium's cruisers are already 5 km in length and the battleships are reported to be "two leagues in length" and that's 9600 meters.

This often feels so much like streamlined, ideal construction times. With numerous assets of any world doing numerous other things, having to wait for various esoteric components to be delivered (read the Rebuilding the Light thread, if you can find it; it gives numerous materials that a ship, even a non-battleship, would need. Any of these may have to be ordered, delivered, fought, negotiated, or bartered over, etc. Various AdMech will need to do other stuff, and any step in ship-making will be dogmatic, ritualized, and SLOWWWWW. Some parts, like the warp engine, I'm not even sure HOW that is made, but Faith and Coin leads me to think I don't want to.

Most important, how long doesn't matter, as the AdMech may not feel they NEED to rush. You aren't going someplace else, and they not only need enough materials to build it, and personnel, but they need enough knowledgeable personnel to MAINTAIN it. There's a lot of Tech-Priests, but they aren't ALL thus initiated and enlightened. Even if they want to build ships quickly, and have the stuff, you can only train the engineers so fast, and each ship needs several to many.

Anyway, numbers don't make much sense in WH40k. Its about the feel you want the setting to have. If you stop to think about numbers, nothing much makes sense in WH40k. For example, hive worlds with 50 billion citizens could raise armies of many hundreds of millions which could make short work of a single chapter of supermarines. Yet entire worlds are allegedly sacked in a single night by space marines.....So just ignore common sense calculations and hand wave the numbers to suit your style and preference.

How often does this happen?

Back in 3rd Edition, planetary campaigns normally took years, often decades or even centuries. Even with a company or chapter of Space Marines involved, they often cut the time period in which the conflict took place much shorter - but they were still long, drawn-out, gruelling conflicts (by comparison to most other sci-fi franchises where a battle for a particular planet might take a couple months - or even shorter). IMHO, the realistic planetary conquest times - from the mind-boggling logistics involved in carrying out such a campaign to the period in which it lasted and the numbers of casualties generated - were one of the best parts of WH40k.

Perhaps it is a euphemism? Space Marines are not, afterall, an occupying force. They are an offensive army suited to crushing one enemy and moving on. Perhaps the alleged instances of overnight Space Marine victories might simply be a reflection of the Space Marine's peculiar talents and point of view: The Space Marines showed up, launched a series of successful decapitation strikes against the planet's rebellious government and orbital strikes against PDF formations and materiél, declared mission accomplished, and left the pacification of the planet's remaining recidivists to the Imperial Guard.

In a sense, the planet was sacked overnight: The structures of governmental control will be in shambles, the governor dead, his highest advisors dead, most of the PDF's generals dead, tanks and war machines destroyed, much of the active duty PDF and most of its leaders dead. To the inhabitants of the planet, they went to sleep under the flag of Chaos and awoke the next morning to find the Imperial Guard, Ministorum, Arbites, and Inquisition landing millions of troops and pre-fab buildings without resistance and the remains of the planetary governor and the highest ranking rulers of the world decorating the walls of the governor's burned out palace and the words, "Sic Semper Traitoris," writ large.

Depending on the length and success of the rebellion, the number of troops necessary to pacify the planet may not even be that great, as members and units of the PDF, local Arbites and Ecclesiarchy, and the population at large may have already been acting in open rebellion against their Chaotic masters, resulting in a minuscule need for outside forces. And thus our hypothetical imperial citizen might awaken to find members of the loyalist factions already patrolling the streets, pulling those suspected of harboring or assisting the traitor government and taking them off to be tried by people's courts, the courts of the local Ecclesiarchy, or the Arbitors.

In any case, the effective result is that the Space Marines sacked the world in a night - even though they didn't actually conquer the world. Conquering it was unnecessary and a waste of precious Chapter resources.

Yeah, even Movie Marines chapter would be hard-pressed; Astartes are the Imperial army that doesn't use the Imperium's superpower, that being limitless numbers. There are just never enough of them, almost regardless of who is writing. Whether there's "a million Space Marines, and over a million worlds", or enough of them that the galaxy doesn't fall apart when whole companies show up, and get wiped out (I think the Nids wpied out most to all of the Ultramarines 1st and 2nd Companies, when they laid siege to McCragge, and the Ultramarines never cried like the Blood Angels did, worried that they were now too few to matter, and needing to raid their Primarch's tomb, while calling in their Successor Chapters for support, or the Raven Guard, when that was legitimate, and it blew up in their faces. Still, they can do heinous damage when they arrive, leave, and allow the Guard to dig in, where then THEY will win via attrition.

Just be happy its not 1 Year per Hull Point Lost = ROTFL