Elevation, distance to the equator, and seasons will influence the current UV index in your area, which will determine ‘time to burn’ the most. The amount of melanin in the skin, and potentially even your antioxidant load can also factor into how quickly you’ll burn.

(Anecdotally, I seemed to burn less quickly when sunbathing for Vitamin D if I took 1000mg of Vitamin C before doing so, based on using a timer. But that could’ve been influenced by a number of unaccounted factors, and would require a proper double blind study to verify).

EDIT: interestingly, there does seem to be some preliminary evidence that ingesting anti-oxidants does, in fact, have an effect on sunburn.