Ok, now how about if some how a ship is left abandoned in space with fuel tanks full and power plant just just idleing like a computer in sleep mode? Or set down on a planet, asteroid, etc. In other words how long will it stay in working order assuming it takes no damage from a meteorite or something else. Assume it is left in full working order and the crew never returned for some reason. How long before the power plant turns to lead or whatever happens to it? Can someone find it after say 200 years and use it? If so, what has to be done to make it usable?
If it's left with fuel tanks full, there needs to be some sort of steady power supply to keep the liquid hydrogen cooled enough to remain liquid (or some other method to keep the ship REALLY cold). Otherwise, if it heats, the hydrogen will go to a gaseous state, and the result will be much like what happens when you put a bunch of water in a pressure cooker and supply enough heat to turn it to steam without giving it a vent...
In other words, BOOM.
A ship could be designed (in theory) to make it easy to keep that fuel liquid, but it's going to require specific design choices. There's going to have to be some assessment of what temperature range the fuel needs to stay at, and what temperature range is going to work for the rest of the ship. What sort of autonomous routines will the ship go to when it's been sitting there? Presumably, there will be standing instructions to "perform actions X, Y, and Z if the following internal conditions exist...", and that's going to be affected by how capable you think ship's computers ought to be.
You could also just have the ship vent the fuel tanks as needed, which might be simpler.
As far as power plant life expectancy... I'm not aware of anything that gives rules for TLs found in Imperial space.
GT:Starships makes some mention
of Darrian ships with extremely long life expectancies, but that seems to imply regular refits and rebuilds. I'd have to say a lot would depend on the situation, but that it's not totally impossible for a ship to "hibernate", so to speak, and be able to fly with some work to get it back into some semblance of functionality. There will be issues with materials that have aged or rotted, but if it's "fly the old ship or die of asphyxiation", you could probably make it workable for ships that are around two or three hundred years old. It's really a GM call, as we don't know the particulars of the technologies involved -- will the power plant crap out? Will the moving parts still move? Will the electronics still work? Will the gravitics handle being shut down for that long? Many materials don't like really long-term exposure to vacuum, too, and we don't know the details of how advanced materials would handle it. If it's not specifically designed to hold up for decades or centuries, I have doubts that it would do so; there would also be the question of whether typical materials in use would have that built in, or if it would only be economically sensible to use materials that aren't excessively durable.
It's not going to be "Hey, cool, look at my spiffy new ship that I found!", but it might be, "Hey, look what I managed to get to move with spit and duct tape and prayer!" You're going to need major refitting at the closest available port before it's even remotely going to be safe.
