It varies by room
Hallways, kitchens, kids' rooms and entryways take constant contact and show wear first. Formal rooms and guest bedrooms can go much longer without looking tired.
Ceilings usually outlast walls by a wide margin unless there's been moisture or smoke.
Signs it's time
Scuffs that no longer wipe clean, visible touch-up patches that don't blend, chalky or fading color, and peeling near trim or baseboards are all clear signals.
Changing the color of a room is a legitimate reason on its own — paint is the cheapest way to make a house feel different.
Prep is what makes it last
Patching, sanding, caulking and priming bare spots do more for the lifespan of a paint job than the brand of paint on the can.