Depends which sim one you are using.
When I had Microsoft Train Simulator (2001) I usually made myself custom freight scenarios that involved a lot of shunting and stuff and could basically go on for quite a bit of time. There's no real limit.
Edit: Also custom tracks could be really really long.
I wish there were an actual flight simulator game, something like ETS2 but with aircraft - to ferry around passengers to earn money for upgrades and new aircraft and all that. Sadly the market is too small for proper developers to consider it, the only thing that comes close are those stupid android games with no real gameplay.
I'd actually be quite interested in something like that. If we add having your own fleet and hiring pilots and stuff, it'd be great. But yeah, not enough demand for such a thing.