To ensure that you get the full SPF of a sunscreen, you need to apply 1 oz – about a shot glass full.  Studies show that most people apply only half to a quarter of that amount, which means the actual SPF they have on their body is lower than advertised.  During a long day at the beach, one person should use around one half to one quarter of an 8 oz. bottle.  Sunscreens should be applied 30 minutes before sun exposure to allow the ingredients to fully bind to the skin.  Reapplication of sunscreen is just as important as putting it on in the first place, so reapply the same amount every two hours.  Sunscreens should be reapplied immediately after swimming, toweling off, or sweating a great deal.