I know it's a no-no, but I usually take one early in the morning starting on CD 9 or so. I can usually feel when I am getting ready to O, so I don't start taking 2-3 a day until either that, or the line starts to get progressivly darker. When I think I am Oing, I usually do 6am, 3pm, and 9pm.
I missed my +OPK last cycle for the first time, and would have missed it again this cycle if I didn't take a 3rd OPK at 10pm.
I could go through 30 OPKs in one cycle if I really wanted to, I love POAS....any stick!