Halakha Kejadian 25:19: Kitzur Shulchan Arukh

If a woman does not have regular menstrual periods, then the thirtieth day¹⁰Beis Yosef (Shulchan Aruch), Bach, Taz, Prisha, Chochmas Adam, etc. after her (last) perception is always considered her fixed period, and is termed "an average onah." For instance, if she perceived the flow on Monday, of the week when the sidrah of Noah is read, then the Wednesday of the week when the sidrah of Toldos is read, is considered her fixed period. In addition, she must also pay attention to the other occasional times of perception, whether based on equal intervening days, or on certain days of the month, as will be explained hereafter, God-willing. Whenever she has reason to expect her period, her husband must separate from her on the onah before, as has been explained.