You should reapply sunscreen every 2 hours when spending time outdoors, especially if you're sweating or swimming—even if the sunscreen is water-resistant. Here’s a more detailed breakdown:
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Every 2 hours: General rule for all sunscreens.
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Immediately after swimming or sweating: Even water-resistant formulas lose effectiveness.
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After towel-drying: Wiping your skin can remove sunscreen.
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Daily use: If you're indoors most of the day, reapplying may only be necessary if you're near windows or go outside.
And don’t forget: apply a generous amount—most people under-apply. About a shot glass full (1 oz) is recommended for the whole body.