For best protection, experts recommend using a minimum SPF sunscreen of 15, applying the proper amount (2mg/cm2 of skin, or about one ounce for full body coverage), and reapplying every 2 hours.
Most people under-apply sunscreens, using ¼ to ½ the amount required. Using half the required amount of sunscreen only provides the square root of the SPF. So, a half application of an SPF 30 sunscreen only provides an effective SPF of 5.5!