2,092. A Pattern of Ten or One Thousand Days
Hilchos Issurei Biah 7:23
If a woman’s pattern is to see blood for nine impure days, followed by nine pure days, she may engage in intimacy for eight days out of every 18 for the rest of her life, as follows: seven of her nine impure days are her niddah days and two are of her zivah days. She must watch one of her nine pure days and she may engage in intimacy on the other eight. She continues in this manner forever.
Hilchos Issurei Biah 7:24
Let’s say that a woman’s pattern is to see blood for ten impure days followed by ten pure days, or any number greater than ten. Even if her pattern is 1,000 days, the following will apply so long as she experiences the same number of pure and impure days. In all such cases, the number of days on which the woman may engage in intimacy are equal her number of days as a zavah. For example, if she has ten impure days, seven of them are niddah days and three are zivah days. On her ten pure days, she counts seven and may engage in intimacy on three. There are therefore three days on which she may be intimate and three zivah days. Similarly, if she’s impure for 100 days and pure for 100 days, the first seven are niddah days and the following 93 start during her zivah days. Therefore, she counts seven of her 100 pure days and she may be intimate on 93 of them. The same rules apply with to a cycle of 1,000 days or any other number larger than ten.