The standard guidance is reapplication every two hours, though several common factors shorten that window meaningfully.
The baseline two-hour rule
Even 'all day' or water-resistant sunscreens are generally recommended for reapplication every two hours during continuous sun exposure — no formulation fully eliminates the need for reapplication over an extended period.
What shortens the interval
Swimming, heavy sweating, and toweling off all remove sunscreen faster than the standard interval accounts for — reapplying immediately after any of these, rather than waiting for the two-hour mark, is the safer default.
Why reapplication matters more than the initial SPF number
A missed reapplication after swimming often does more to reduce actual protection than the difference between SPF 30 and SPF 50 applied once — reapplication discipline is arguably the bigger lever most people underuse.
Does sunscreen expire?
Yes — most sunscreens have a shelf life of about 2-3 years, and expired product loses effectiveness; checking the expiration date (or the small open-jar symbol indicating months-after-opening) is worth doing before a big sun day.