I don't have details at hand to cite, but I recall encountering some recent news of research that may place in question equating a simple measure of sun exposure to elevated risk for skin cancer. (I didn't look at in depth, and my memory could be somewhat faulty.)
It may not be simple quantity of exposure, but rather severe sunburns that most elevate risk. If one is outside regularly and acclimatized, these severe burns are less likely to occur.
Perhaps someone else will have a more detailed comment on this.
Of course, one's pigmentation will also influence the likelihood of sunburn, although dark pigmentation does not entirely prevent burning.
EDIT: Just read InclinedPlane's comment, above, which mentions the same concern with sunburn.
It may not be simple quantity of exposure, but rather severe sunburns that most elevate risk. If one is outside regularly and acclimatized, these severe burns are less likely to occur.
Perhaps someone else will have a more detailed comment on this.
Of course, one's pigmentation will also influence the likelihood of sunburn, although dark pigmentation does not entirely prevent burning.
EDIT: Just read InclinedPlane's comment, above, which mentions the same concern with sunburn.