Find how long your sunscreen lasts by SPF.
SPF multiplies the time to burn, but only at the laboratory application density of 2 mg per square centimetre — most people apply roughly half that, and protection scales as SPF raised to the power of the fraction applied, not linearly. That is why a thinly applied SPF 30 behaves like SPF 5 or 6. Whatever the arithmetic says, reapplication is capped at two hours, or around eighty minutes when swimming or sweating.
Effective SPF and reapplication
Effective SPF = SPF^(thoroughness/100); protection minutes = min(unprotected burn time x effective SPF, 120 min, or 80 min if swimming)
Sunscreen is one part of sun protection and does not make unlimited exposure safe. Shade, clothing and avoiding peak hours matter more. Seek dermatological advice if you have a history of skin cancer or photosensitivity.
In theory yes, in practice barely — SPF 30 blocks about 97% of UVB and SPF 50 about 98%. Reapplication frequency matters far more than the number on the bottle.
About 35 ml — roughly a shot glass — for a whole adult body, and a generous half-teaspoon for the face and neck. Most people apply a quarter to a half of that.
Every two hours, and immediately after swimming, towelling or heavy sweating regardless of water-resistance claims.