Unless there is a paint defect from the factory-paint should last the life of the vehicle with proper maintenance. That's not to say there isn't adhesion/brittle paint problems (i.e. rock chips) with aqueous based paint-but over all it should last.
In my experience owning vehicles, 1980's and 1990's paint held up WAY better than what's on cars today. The white on my '96 Maxima takes a pick-axe to damage it (slightly exaggerating). My '96 Grand Prix was the same way, other than very light clear coat oxidation in places, it looked awesome 25 years into its life. I know plenty of 90's paints had issues (white on Chrysler vehicles was particularly bad), but many of them were adherence to aluminum hoods which was new at the time. But durability on the old paints can't be beat.
You know you got me. I forgot about the 20plus year old beaters on here. Yea- after twenty years paint would probably be tired. Especially if your an owner of a vehicle that has had numerous previous owners.
