Up to 6 hours of natural protection against mosquitoes Mosquito repellent spray can prevent bites from mosquitoes and horseflies. After treatment, the mosquitoes can no longer identify the person as a target. Generally, you are protected against mosquitoes for up to 6 hours after application. However, if you are engaged in strenuous activity that causes sweating, the protection may only last 3 to 4 hours.