A380900: Modern epoxy paints have a pretty good lifespan, but I'd say most airline aircraft will get painted somewhere between 5 and 10 years. And they get washed on the order of every 6 months. It's more of a "corporate image" issue than performance. Of course, as individual aircraft change owners, and companies change logos, this will cause repaints to occur.
Aircraft do not always get stripped to bare metal. Many operators choose to sand off the topcoat and retain the primer, electrostatic undercoats, antenna planes, etc., and then reapply a new topcoat. Since the topcoat should be about 2 mils thick (in the case of one popular brand of paint), it doesn't take a lot of sanding to remove it. The composite parts are also usually sanded, as some strippers will attack the material. Regards,