This is how it went for me this cycle:
CD14 CBFM - LOW
OPK went + late that night around 10pm. Had I gone to be sooner, I would have missed it as I tested frequently during the day.
CD15 CBFM - PEAK
OPK +. All day.
We DTD that night at 10.20pm exactly - yes I checked the time LOL
So around 24 hrs after the start of my LH surge on the opk.
Today, CD16 OPK has been + but fading out this afternoon/tonight.