Generally speaking, the factor number of the SPF on the sunscreen refers to the amount you can multiply your sun-exposure with. For example, factor 10, should allow you to stay in the sun 10 times as long as with no sunscreen. Having said that, the sun is warm and you will wear off the sunscreen by sweating and it needs to be reapplied.
Although you should wear sunscreen to prevent burns…....there are those who believe some unprotected exposure is healthy….. http://www.usatoday.com/news/health/yourhealth/2006–05-07-your-health-sun_x.htm