Sunscreen re-application is as important as initial application. Sunscreen lotion should be applied 30 minutes prior to be exposed to the sun, and usually lasts for about 2 hours once on the skin. And so, it is important to reapply sun protection throughout the day to ensure its full effect. Carrying a non toxic sunscreen mist like the Anthelios Mist from La Roche-Posay is perfect in this regard as you can carry it with you throughout the day, touching up as needed. Sunscreen, once opened, generally can last up to 3 years until its protection value lessens.