Hi LG! How are you? My cycle is normally 28 days, and I start using OPK on day 10 or so. This month I got a faint positive on CD12, faint on CD 14, but reallllyyy strong on CD15. Last month as soon as i got a faint line I took that as a positive, but now I think you have to wait until the line is strong as the control line. I usually do it at around mid morning, and again late night, just to be sure.
Good luck hun.