What is the 'reset time' on sun exposure as related to sunburn?

Sunscreens and sun exposure advice is usually given in terms of how long you can be exposed without suffering 'burn' damage. Add a sunscreen and the exposure length increases by a factor 4, 8 , 50 etc. But no advice I have ever seen gives a sample rest time between exposures.

If I go out at midday on a sunny day in Texas and spend 30 minutes exposed, and then go back inside again, how long do I wait before I can expose for another 30 minutes without suffering the approximate equivalent of 60 minutes exposure?