SPF50 should protect you 50 times as long as your skin would be able to naturally protect itself. However, remember that the lower the SPF and longer the length of time since application, the less UVB rays will be filtered, so a reapplication every two hours is highly recommended.

Regardless of the SPF rating (whether it is SPF 30 or SPF 50), sunscreen only remains effective on the face for approximately two hours after application. Many people mistakenly believe that a higher SPF number means it lasts longer; in reality, the number only indicates the level of protection against UVB rays (SPF 50 filters about 98% of UVB, while SPF 30 filters about 97%). The physical and chemical barriers in sunscreen break down over time when exposed to direct UV light. Furthermore, natural oils from your skin, sweating, and touching your face rub the product away. If you are swimming or sweating heavily, the protection time drops significantly, and you should reapply immediately after towel-drying or every 40 to 80 minutes as indicated by the "water-resistant" label. For daily wear under makeup, experts suggest using a powder or spray SPF for touch-ups throughout the day to ensure consistent coverage and prevent the cumulative sun damage that leads to aging and skin cancer.