How frequently should you apply sunscreen? It depends on the kind of sunblock, how sensitive your skin is, and what actions you'll be doing. You should apply sunscreen about half an hour before going outside. Sunscreen works greatest when it's applied before going out into the sun and then again after 15 to 30 minutes of sun exposure. It depends on your sunscreen. It is of high SPF then it is not necessary to reapply sunscreen after 15 to 30 minutes. If your sunscreen is water resistant then it is not necessary to reply sunscreen in short time.