I guess "whenever you're going to be out in the sun for an hour or more" is the best answer for me. I often am working outside in the sun for the whole day, and shade is not always easy to come by (even portable shade). So I put on GOOD sunscreen in the morning, and usually reapply during my lunch break. If I don't do this, I get sunburnt (those days when I forget to bring sunscreen, or run out by accident for example).