[njarboe]

People's own feeling on how the sun feels on their skin without sunscreen might be a great indicator of how good it is for you and an individual. Most people don't pay much attention to that, I would guess, but as a person with fairly light complexion (German, French, English, Irish, + minor unknown other) that grew up in sunny northern New Mexico, I find that the direct sun on a summer day feels good for just 5-15 minutes, while on a cold winter day I enjoy it much longer.

[StavrosK]

I agree, that's what I go by as well. After a few minutes it becomes uncomfortable in the summer, and I look for shade. The only exception is when you're swimming in the sea, when you don't really feel the sun's rays.