If I am not pregnant when will...
You will need to know when you ovulated...
10 days after ovulation your basal temperature will begin to drop as your body realizes you you are not pregnant.
This basal temperature chart shows a drop in temp at ovulation (Day 11) then a rise the next day, then another rise day 15, then a significant rise day 23.
If conception and implantation did not occur in a cycle, the Basal Body Temperature will drop near or below coverline the day before or the day of the start of menstruation. This is due to the degradation of the corpus luteum and the subsequent drop in progesterone levels.