The ingredients of the sunscreen and how much you use, will determine the amount of UV protection you get. Zinc and titanium based products tend to give you wider coverage and last longer on the skin. Avobenzone products tend to disintegrate within two hours in the sun, so it is important to reapply every two hours.
The amount you put on your body is very important. A teaspoon for your face and a shot-glass full for your body, will make sure that you have enough sunscreen to protect you from UV light. The ingredients in the sunscreen, as well as the amount you put on your body, are very important in determining how much protection you will get from UV rays.