35-37d Cycle Ladies??

Stacha
My cycle is around 35-37d. I'm currently on cd21 and I've had + opk 3 days in a row... With cycles this long & because of not knowing my exact O day (since I'm going on 3 days now), when should I start taking hpt? Too anxious to hold off for my TWW. Anyone with a similar cycle? How soon did you take a hpt? ...mainly asking to those who received a + hpt...