It also depends where you live - Cancer Council recommendations are that in Victoria, children don't have any sun protection at all betwee 1st may and 1st September. That means no sunscreen, no hats, but also minimise exposure between 10 and 2. This advice was generated based on cases of rickets, caused by low Vitamin D.

In QLD, however, the recommendation is to use protection all year round. I guess it's something to do with intensity or brightness of the sun?

For grown ups the advice is 10-15 minutes per day on the arms, hands and face. Prefereably early morning as UV levels are not as intense then.