Letrozole 2-6 when to test with opk?

So I took letrozole cd 2-4 so far I have 2 more days when do you think I should start testing for ovulation? I’ve never taken letrozole or ever ovulated before and I have lots of folicles about 37.. im worried they are going to drop fast and im going to miss my surge. So When do you think I should start testing for ovulation?