Sunscreen (also known as sunblock, suncream, suntan lotion) is a lotion, spray or other topical product that helps protect the skin from the sun's ultraviolet radiation, and which reduces sunburn and other skin damage, ultimately leading to a lower risk of skin cancer. Exactly how much to apply can be estimated based on how much skin is exposed. The common advice is that sunscreen should be reapplied every 2~3 hours; however research has shown that the best protection is achieved by application roughly 15~30 minutes before exposure, followed by one reapplication 15~30 minutes after the sun exposure begins. Further reapplication is only necessary after activities such as swimming. In caring for children's skin, do the following: Use plenty of sunscreen with an SPF of at least 15, every time your child goes outside. Apply it at least 30 minutes before heading out and use it even on cloudy days. Apply a thick dose and remember to cover the backs of knees, ears, under the eyes, and the neck and scalp. Reapply every two hours.
It depends on the kind and type of sunscreen
Normally it will tell you on the container