Hi, so I am fairly new to this RPG and only played Pathfinder before (for 2 years now). I am familiar with the W40k universe though and so I am looking forward to be the GM for a group of some of my friends. I looked around a bit and found out that there are pre written adventures. Which is very nice, since I do not think I am able to write something own at this early stage. So my question is: How long does it take to play e.g. "Purge the Unclean" and how far will the PC' s level? Are there pre-written bigger campaigns that can be played on several evenings (more than 3 or 4) for Dark Heresy?
Thanks for your help.

The time is relative to your GM style and how fast the pc's pick up on the plot of the adventure. I'd say anywhere between 3-6 good length sessions, good length (a few hours) sessions per part of Purge the Unclean. Your mileage may vary of course depending on if the group gets sidetracked.

Just remember that if they do get sidetracked that's ok, just try to make sure they don't get bored of what they were doing and redirect them in an inconspicuous way.

Leveling is dependent on the time taken and how generous you are with xp. I give out between 100-200xp per session depending on how much they got done. You may want to give them more or less. They should have a recommendation or relative rank the pcs should be at.

If you want a long campaign then the Haarlock Trilogy is considered one of the better Dark Heresy trilogies avaliable, it has a smaller adventure in the back of Disciples of the Dark Gods as well that you may want to use too.

I always recommend starting with Edge of Darkness, available from the FFG website

You might have a hard time getting "freebee campaigns" in the web, so. As you know yourself, it is cumbersome to come up with something like that..and ten times more so to put it down into a PDF or Forum post

Anway, you will find further adventures/missions in the old DH Support section. Once, there was an adventure writing competition and the finalists were put up for download.

I always recommend starting with Edge of Darkness, available from the FFG website

I second this. Edge of Darkness (available from the Support section of the main DH page) makes a much better introduction to Dark Heresy than the sample mission in the back of the rulebook or any other published adventure.

Beyond that, there is an older Thread somewhere on these Forums spelling out a good order to play the published adventures in. I can never get the Search function to work, but if you want to try, the Thread was titled 'Order of Published Adventures?' or something like that.

+1 vote for Edge of Darkness, it is such a great scenario.

As for searching these forums, Google works just fine Just add the switch: -site:fantasyflightgames.com

Personally I always make my own senarios because it gives me a greater understanding of the plot and NPCs, since I thought all of it up myself. You can definietly start with a prewritten adventure, and use it to introduce yourself and your players to the setting of warhammer 40k.

Afterwards I recommend you to make your own plots and adventures using the rulebooks descriptions as inspiration, or a prewritten adventure where you change it on your own whim to make it personal for both you and your group. The reason to this is that it'll give you more experience and control of the game, which will help GM'ing the game

I'm not the biggest warhammer 40k nerd myself, so I usually depict the setting in my own view of it. The lore is vast and even contrary to itself sometimes, so you're better of with your own interpretation than trying to keep taps on way too many sources.