I O between CD 13-CD 15. I generally start my OPKs on CD 10.
Last cycle I did two every day from CD 11 through CD 14. Then I did one on CD 15 & one on CD 16 because they were clearly negative at that point.
This cycle I only did one per day until the day I got my positive. I was running low on OPKs and I didn't want to run out. I tested twice the day I got my positive... mostly because my arsenal was replenished that day!
I know some girls use FMU, but I generally won't test until at least 11:30 in the morning.