A sunscreen with SPF 30 will protect you from around 96.7% of UVB rays, whereas an SPF of 50 means protection from about 98% of UVB rays. Anything beyond SPF 50 makes very little difference in terms of risk of sun damage, and no sunscreens offer 100% protection from UVB rays.
If you just go to work everyday and spend most of your time indoors, it is recommended to choose SPF 30 sunscreen. You do not need to reapply for indoor activities. Apply it before going out in the morning to protect you all day. For outdoor activities, such as swimming, climbing and other sports activities, it is recommended to choose SPF 50 , Putting on more sunscreen every two hours helps keep you protected. It’s important to remember to re-apply after swimming or water sports also.