It really depends on where the truck will be stored.
Outdoors, anything will eventually weather and fade. That process is faster depending on the climate, proximity to the sea/salt air, and whether its parked under trees, etc.
Indoors, pretty much anything will last forever if you take care of it.
I have Krylon rattle-can on my truck right now and its been mostly outside for the last 3 years. It has faded faster than a gun applied finish, but it still seems to be holding up fairly well. That said, I did sand and prime with an etching primer applied with a gun first, then sprayed the Krylon over top of that, so I suspect that is why its lasted as long as it has. It also is very easy and quick to touch up too.
Prep of the surface goes further than anything else though, as far as paint longevity.
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