About two hours of outdoor time, then put it on again. That interval is the AAD reapplication rule, not a guess. One morning coat will not carry a beach afternoon. Sweat, water, and a towel remove product faster than the label implies.

The two-hour rule is about film, not SPF

Use the same amount as the first coat: about one ounce for the body, half a teaspoon for the face. SPF 50 does not stretch the clock versus SPF 30. The SPF digit is how much UVB the intact film blocks, not how long the film stays on you.

“Water-resistant 80 minutes” means the product passed an FDA water test for that window. It does not mean you skip the towel-dry coat. Reapply when you get out.

What actually wipes it off

• Swimming or a hard sweat—often under an hour of useful film
• Toweling off
• Rubbing against a shirt, car seat, or backpack strap
• Chemical filters that break down in strong UV (avobenzone is the usual example)

Mineral filters are more photostable on skin, but sweat and fabric still scrape them off. Indoor days next to a window can stretch to three or four hours; a hike should not.

Pair the timer with today’s peak on the homepage. A High or Extreme afternoon is a bad time to “remember later.”