Sunscreen should generally be reapplied every two hours to maintain effective protection against UV rays. This applies especially if you are outdoors, sweating, swimming, or towel drying, as these activities can reduce the sunscreen's effectiveness

. If you are indoors and away from windows, you may not need to reapply as frequently, possibly every four to six hours, but if you are near windows or stepping outside often, reapplication every two hours is still recommended

. In situations involving prolonged outdoor activity, such as at the beach or during sports, reapplying more frequently-every 40 to 80 minutes-may be necessary because sunscreen breaks down faster with sweat and water exposure

. Key points for reapplying sunscreen:

• Reapply every two hours during regular outdoor exposure.
• Reapply more frequently (every 40-80 minutes) if swimming, sweating, or towel drying.
• Use enough sunscreen (about one ounce for the entire body) for full coverage.
• Don’t rely on high SPF to extend the time between applications; all sunscreens need regular reapplication

In summary, reapplying sunscreen every two hours is the standard recommendation, with shorter intervals needed during water activities or heavy sweating to ensure continuous protection.