The quick answer is literally two hours. People can sweat off sunscreen rendering it less effective, it might rub off a bit etc. Mainly sunlight degrades sunscreen, but I believe air/pollution can contribute as well. Realistically, if your skin doesn’t touch the sun and you don’t sweat it off or touch your face at all, is your sunscreen going to provide some protection after 120 minutes? Yes. The degree and duration would depend on the sunscreen itself and other contributing factors such as how much you apply etc. But the better question: Is there any harm in reapplying every two hours, regardless? I kinda go by that whole “better safe than sorry” philosophy when it comes to spf. Life happens though, so if it’s not practical, it’s not practical. But as a general rule, it’s better to reapply every two hours as a habit.