Most sunscreens with a Sun Protection Factor of 20 or higher do a sufficient job of protecting your skin against UVB rays. SPF is a measure of a sunscreen's ability to prevent UVB from penetrating and damaging your skin. Here is how the whole process actually works:

It may take 20 minutes for your unprotected skin to start turning red and to get a sunburn. However, using an SPF 20 sunscreen theoretically improves the time it would take to burn your sun by 20 times. Of course these are values measured in labs and in real life you are looking at values between 10 and 15 times to be more realistic and safe.

Another way to look at it is in terms of actual percentages:

A Sunblock with a SPF 15 rating blocks approximately 93 percent of all incoming UVB rays. A sunblock product with a SPF 30 rating blocks approx. 97 percent; and SPF 50 rated product blocks about 98 percent. These may seem like very small differences, but if your skin is extremely light-sensitive, or if you or a close family member have a known history of skin cancer, those extra percentages will make a signficant difference. However, as you can see, no sunscreen can block all UV rays and keep in mind that all these ratings are developed in a perfect environment. Water, Sweat, or just different amounts of sunblock on your skin will create a different environment.

Also, keep in mind that no sunscreen, regardless of the advertised strength, should be expected to stay effective for more than two hours unless another layer of the sunblock is put on your skin. Second, "reddening" of the skin is a reaction to UVB rays alone and tells you little about what the UVA damage is that you may be getting at the same time. Your skin can get plenty of damage without the red sunburn showing on your skin.