Well, seriously depends on the way u treat your car, how often u wash and stuff like that, my original paint lasted me nearly 7 years from the factory. Though i'll wash it once every week(sometimes few times a week if it's really dusty and dirty, but sometimes 2 weeks once due to myself being busy and there was no time to wash), wash wif car shampoo every 2 weeks, wax every month, polish once a year.
Only recently get it repainted on the outside layer due to a lousy job(the paint chipped more than usual from the pebble hitting) by the workshop many years ago on the bonnet(accident, claim issurance fix, lousy workmanship coz not my regular painter), and also some scratches here and there thanks to the kids in schools walking past wif their schoolbags hitting the car here and there when i go fetch my bro. And the fact that well, it's 7 years already, what the heck, can send for repaint edi lar...
Well, since i was repainting, took the chance to add quite a lot of parts onto the car, exterior wise. And well, quite satisfied with it, although the colour seems to be of a ligther shade than what my car used to be.