Good question. Inexperience is a culprit. For example, it may takes several months for you to be able to distinguish from “fertile” vs. “infertile” mucus. If you miss a day of temps, there is a chance you may miss a temp shift. Or, it may be irregular signs, such as flucctuating temps that don’t follow the models closely. These things take time to learn how to read properly. Most if this will fall on the wife to be able learn her body, her signs, how her temps and other signs change and relater to her phases…the husband gets off kinda easy…just hands the thermometer over and practices self-control.
Until you really learn to chart and what the chart is telling you, there will be longer periods of abstinence. But once you get the hang of it, unless your phases are irregular or there are other hormone issues at play, abstinence can be a 1-2 week window…it varies from couple to couple. I do know a couple who has had to abstain for MONTHS due to irregular charts. But, their situation would be grave if she got pregnant, so they stick it out…“for better or for worse…in sickness and in health…”