The effect of Listerine is supposed to lasts for at least three hours. However, Listerine actually hasn't been tested as an insect repellent but since it contains eucalyptus and menthol essential oils, it can be used as an insect repellent. Insects such as flies, fleas, mosquitoes, cockroaches, and other bugs don't like the smell of eucalyptus and menthol so they won't be that much attracted to you if you apply some Listerine on your skin.