BBN is believed to have started a few tenths of a second after the big bang when temperatures were roughly 116 GK, and lasted until the universe had cooled to about 1.16 GK. According to the FLRW metric, this was about 16 minutes. I get the impression that the duration is important, but don't understand how. Are the relative abundances of elements dependent on the temperature difference (i.e. BBN starts at 116 GK when stable nucleus can form and ends at 1.16 GK when they are frozen), or is it time dependent (e.g. it starts at 116 GK and goes for 16 minutes after which time no more nuclei can be formed because the raw materials are used up)? Or is it volume dependent (e.g. It begins forming at 116 GK and continues as long as the volume of space expands at some rate)?