If your cycle has been longer than thirty-one days at least three times in a row, then you do not have to observe the Hebrew date the following month. It always falls within the time period during which you have an established pattern not to menstruate.

There is debate as to whether you also are exempt from keeping the thirtieth day. On the one hand, the logic is similar to the Hebrew date. On the other hand, the halachot of this onah are more stringent.

We follow the opinion that a woman who has had a longer cycle at least three times in a row is required to observe only the interval onah. Should she menstruate on day thirty or earlier on any given month, she would have to observe all the onot the following month and until she reverted to her longer cycle.