I think my wife and I typically have to abstain for about 12 to 14 days in the middle of my wife’s cycle, though it can occasionally be as long as 20 days or even a bit longer. But in those cycles when the abstinence time is longer, that is because that entire cycle is longer (e.g., 36 or 40 days total instead of about 29 days), so the number of non-abstinence days per cycle usually doesn’t change much at all. (For the record, we use the Billings Ovulation Method.)
You mentioned that you have talked to three different NFP coaches. Were these coaches all teaching/coaching the same method of NFP? If so, you might want to try a different method, to see if you get better results. There are several NFP methods, and some methods work better for some couples, while other methods work better for other couples. The major methods that I know of include the Sympto-Thermal Method (STM), Billings, Creighton, and Marquette.
Here is a post where I elaborated a bit on the different methods.
It is possible that your fertility signs simply aren’t as easy to interpret as most people’s, and that therefore you simply will have to abstain much longer than most couples. But if I were you, I wouldn’t accept that answer until I had tried at least two or three different NFP methods (as taught by qualified instructors). You also could visit a doctor who uses NaPro Technology (a pro-life, morally sound method of treating fertility problems), to check if there is any medical problem which could be making your fertility signs difficult to interpret. (You can find a doctor at
www.fertilitycare.org.)