When I took prometrium after O, it definately delayed AF. What I did was take an early pregnancy test about 10 days post ovulation, and if bfn, stopped the prometrium. Not a super scientific method I know, but I found that the early FRER's with first morning urine were pretty accurate at 10 days. I then found I got AF about CD 28 or so. Hope that helps a bit...
I found that progesterone delays my AF and usually my ovulation the following cycle was earlier to put me back on track! It usually took 1-2 days for AF to arrive and then ovulate was 2 days earlier, so for me, it seems my body still wants to same on the same cycle.
Risa