Damaging high doses can be as little as a few minutes in the Australian summer so I think it's fair to keep that in mind.
On your evolution comment, most Australians don't have dark skin, but the ones who were here before Europeans do. I think that is a good indicator that the body is adjusting to something harmful. On the other hand, many dark skinned people in Britain aren't in a climate they evolved for either. We all have to be mindful that people move around the world faster than evolution now and may need to adjust our habits.