There is no one set number
Some homes need paint sooner than others. A shaded inland wall may age slowly. A beachside wall in full sun may fade much faster.
Prep has a big effect
Paint can only hold as well as the surface below it. Dirt, chalk, loose paint, bad caulk, and trapped water can shorten the life of a new finish.
Watch the sunny sides
South and west walls often get the hardest sun. Color fade, dry caulk, and hairline cracks may show there first.
Look for warning signs
Chalk on your hand, peeling edges, open cracks, bare spots, and water marks are signs that the wall should be checked.
Do not wait for major failure
Small prep work is often easier before large sheets of paint start to peel. A yearly walk around the home can help you catch changes early.
What to do next
Check the sunny sides first. Take photos of fade, chalk, cracks, and peeling edges.