The Sun, It Burns

To what extent do the effects of the Sun’s rays aggregate in your skin to produce a sunburn? For example, if I go out in the full sun I will achieve noticeable erythema within about twenty minutes as I have a complexion roughly equivalent to that of a trout’s belly. If, instead, I go back inside after only fifteen minutes, how long will I have to remain beyond the reach of the Sun’s unforgiving rays before I can go back out and stay another fifteen minutes without burning?