Generally, we're 2-4 times a month people (which we're both happy with) so we def had to ramp it up when we were TTC. I charted (temps, CM, OPKs, the works, read "Taking Charge of Your Fertility" for details). I forget what we did the first couple months, but once I had my ovulation "range" (I think it was like day 12-20, it was pretty wide) we had sex every other day starting four or five days before my earliest ovulation time, then when the OPK was positive, we had sex every day up to and including the day after ovulation temp spike, and then one more time the next day or the day after. So on the short end, we'd have sex on days 7, 9, 11, get a positive OPK that day, 12, see the temp spike, 13, then day 14 or 15 for good measure. On long months, it's be days 7,9,11,13,15,17,19, get a positive OPK, 20, see a temp spike, 21, then day 22 or 23.
That was def all the sex we had for the 7 months we were trying.