 So an SPF of 30 might last two to three hours before it starts to wear out, so to speak. Okay. So an SPF of 50 might be four to five hours. So if you're going to be at the lake or you're going to be hiking for seven hours, you might have to reapply. It depends on the SPF. Start with 30 at the minimum. 50 is a good medium right there, SPF 50. So just because you're wearing SPF 100 doesn't mean you're 100% covered for the day.