Has it been listed anywhere how long it takes to install a vehicle attachment?

Not so far as I know.

IMO, this is a perfect example of a case where the GM should figure out what works best for their story, and then go with it. And if someone has a talent that lets them reduce the amount of time it takes, then make that an important part of the story.

So, the $PARTY would have failed $MISSION, if $PC didn’t have $TALENT that sped up the amount of time it would normally take to install $ATTACHMENT.

Has it been listed anywhere how long it takes to install a vehicle attachment?

Nope. SWAG applies imo.

Repairing hull trauma takes days if not weeks if there is a lot of damage. As a rule of thumb I would say that installing a 0 hard point attachment takes a couple of days and a couple of weeks for 1-2 hard point attachments. If the proper facilities are available.

Technically speaking, the rule book does say that all attachments (even ones for vehicles) just take a few minutes of uninterrupted work to install.

That said, I don't think you'd get any sideways looks if you told your players that installing that new quadlaser on their freighter will take an hour or so and require heavy lifting equipment (available and included in the docking fees in most modern spaceports, but probably not that dirt strip out at Anchorhead).

Crafting,repairs, and mod timetables are one of those things that typically don't need a hard set timetable. Either you're doing it as between adventure bookeeping, where time doesnt' matter, or you're doing it as a component of an actual adventure with the time being a variable with actual consequences set by the GM.

It took a month to install the Retrofitted Hangerbay on our PC's ship.

I don't pay much attention to listed time for most ship stuff, be it repairs or installing mods. I go with what sounds logical. A relatively small or non-invasive sounding mod should be fairly quick. Hours or a couple days at most. Something more intrusive, such as adding hangars to a ship totally not designed for them, might take much longer. Possibly weeks.

On a related note, I don't pay much attention to listed repair times. There's no way I'm going to make the characters have their ship offline for weeks or months for repairs. It becomes an impediment to the game. From a nuts and bolts standpoint it also just seems ridiculous that repairing a vehicle can take so long, and then installing rather intrusive non standard custom feature takes only days.

I have suggested that it would take a couple hours to install each attachment on thier cloud car (as per Jewel of Yavin), and then the extra time for each mod. But I will allow them to reduce this time by making mechanics checks for installation.

Does this sound fair?

I think reasonableness is the benchmark. What's reasonable. New tires on a car is not a big deal, as wheels are designed to be replaceable externally. Pull and engine and put a new one in? Not so much. So what is the attachment and the mod to it, then ask what is reasonable and how invasive.

Slapping a new weapon on an empty HP? Probably a day job. Like kaosoe's example, making hangar space equals modifying the super structure of the ship to a certain degree, probably not so quick.

It really depends on what the mod is. A software upgrade to the ship's gun to install Targeting Computer (or whatever the hell the Upgrade Your Roll mod is) - a few moments. Upgrading the hyperdrive to go from a x2 to a x.5? That'll take you offline for a week, easy.