Generally youll ovulate 12-36 hours after a positive OPK (its not guaranteed though, all an OPK can tell you is that your body is gearing up to ovulate, the best way to confirm it is to track your BBT).
You dont have to wait for it to turn negative though. A negative is anything other than the line being as darker or darker than the control line. Some people will have a faint second line all the time.