How often is the most optimal for a couple to have sex when the woman (31y)has pcos (normal weight but irregular/seldom Menstruation) to Get pregnant?

Hard to say. The difficulty when trying to conceive with PCOS is that ovulation happens very irregularly (or not at all). So the timing of your intercourse is probably not the issue here. Conceiving is a very time-sensitive issue. Once you ovulate you have only a 24-36 hour window to conceive. You would be better off seeing your doctor, being placed on meds (like Clomid), and being monitored with sonograms.