According to Chimento, a general rule of thumb is to reapply bug spray that contains 20% or more DEET every six hours. On the other hand, a spray that contains 7% DEET will provide 90 minutes of protection, she adds. “Check the label and look at the concentrations of the active ingredients,” Chimento explains. “The higher the concentration of the active ingredient, the longer the protection and the less you need to reapply.”  And if your outdoor activities involve water, Chimento says you’re going to need to reapply your insect repellent more frequently. “Bug sprays can wash off in the water, so you should reapply afterward if you are sweating profusely or swimming,” she says.