You can only have mulitple ovulation within a 24 hour time period (this is not an issue for NFP methods). A woman can have more than one follicular wave (1-3 waves), with only one wave being ovulatory. The rest will be annovulatory (no ovulation), this how a woman might have more than one cervical mucous patch. This has been proven by many research studies dealing with infertility, usually only one wave is ovulatory & the possible multiple ovulation only occurs within a 24 hour window (not days or weeks).
All NFP methods teach how to handle the possibility of more than one CM patch and these are highly effective. If the rules are applied correctly then NFP is still as or more effective than the highly touted ABC methods (like the Pill).
IF a woman is finding she believes she is fertile the entire month even with cross-checks (such as OPK tests) then she should contact her NFP instructor or find a NFP doctor. There could be a misapplication of one or more of the cross-checks she is using or something else that is being missed that could be rectified & make the method clearer to use.
There is also the possibility of using a STM method, which uses CM & Basal Body Temp. This might be another option, if a woman wants another cross check.
Personally I use the Marquette Model (from the Marquette School of Nursing), while it was a little more costly up front for the Clear Blue Easy Monitor it helped cut out confusion & CM is still my primary indicator. It’s also put DH more at ease & that helps us in other ways;) A monitor has been clearer for me than simply using the individual test sticks & trying to read them myself.
I voted 15-22 days, depending on the length of the cycle (28-32 days). We normally don’t “embrace” during menses which would put it closer to the 15-17 day mark. We use the S.P.I.C.E. approach when abstaining.