Two hours is the rule, but window light changes things.

Every two hours if you’re outside, even if you don’t feel sweaty. UV exposure is cumulative and invisible. You don’t need to be baking on a beach to get burned.

If you’re indoors but near windows (especially uncoated glass), that counts too. UVA goes through glass, causes aging and skin damage, and adds up over time. So if you work by a window all day, reapply at least midday.

The two-hour rule exists because sunscreen ingredients break down with UV exposure, not just sweat or water. Even if you’re bone dry, the protection degrades. Set a timer if you have to.

Your skin doesn’t care if you think reapplying is annoying.