People that TAN i got a question....?

so iv been tanning for about 2 weeks for the frist week i went to quicky and got pink from being in the beds for a little to long(15 min) the max is 20. so i didnt tan for a couple days. (by the way im not very pale or anything i tan pretty well) and yesterday i went for 13 and was fine. and today i also went for 13 and i still seem just fine..now im going tomorrow and i dont know if i should up my time to just 14 or keep it the same. i really dont want to burn at all.