Reapply every 2 hours near a bright window.

Same rule as outdoors: every two hours if you’re within arm’s reach of a window getting direct sunlight. Windows block most UVB (the kind that burns quickly) but let through UVA (the kind that ages skin and raises cancer risk). If you’re sitting next to a south-facing window for three hours, you’re accumulating real exposure.

If the window has UV-blocking film or you’re more than a few feet away, you can probably stretch to every 3–4 hours. Cloudy day or tinted glass? You’re fine with one application in the morning. But if you’re paranoid about skin damage, reapply at lunch and call it done.

Better to be the person who oversunscreens than the person who realized too late.