Mosquito repellents can last anywhere from a few hours to days at a time, depending on what repellent you use. For example, DEET repellents are available in a variety of concentrations that indicate how long the product will be effective. If you’re looking to reduce your exposure to chemicals, using a lower concentration of DEET might be best; however, you’ll have to reapply it more often for it to be most effective. The best course of action is to check your mosquito repellent container for information on how often you should apply it.