SPF is short for Sun Protection Factor. To figure out how long sunscreen will last once it's applied, take the amount of time it would take to burn without sunscreen (UV Index) and multiply it by the sunscreen's SPF. This number will tell you how long you can be outside without sunscreen.
For example, if it will take you 10-minutes to burn today without sunscreen, and you used an SPF 12 (10x12=120), it would take your two hours or 120-minutes to burn.
Keep in mind that SPF only serves as a guideline. How sunscreen affects and protects you depends on your skin type, the strength of sunlight and the type of sunscreen you use (gel, cream, lotion or oil). It also depends on the amount you're using and how often you reapply it, especially if you've been sweating or swimming.
