If a woman had a menstrual flow that had not ceased for 12 years, how strictly would the rule of Lev. 15:25-30 be enforced in terms of social separation required for such a woman? Would the law be relaxed for a woman when it involved years of ritual impurity, so that she could resume at least some normal duties of a wife and mother? I am also interested in an answer that would reflect views in the late Second Temple period.
The Gemara in Nidda 54A discusses a situation like this. Although the Gemara says it’s highly unlikely, the situation is such that the woman has a menstrual flow every three days for three days, (or other such type of situations, see the Gemara there). This means she will never have seven clean days.
The Gemara says that once this cycle sets in, she will be forbidden forever.
שלשה ימים טמאין ושלשה ימים טהורין משמשת שני ימים ושוב אינה משמשת לעולם
If out of every set of six days, three days she is impure, and three days she is pure, she may engage in intercourse with her husband during two of the days, and thereafter she may never again engage in intercourse with him. The eighth and ninth days, which are the first days of the flow of ziva, are the second and third of the three days on which she experiences bleeding. Therefore, she can immerse and purify herself on the night of the eleventh, and engage in intercourse with her husband on the eleventh and twelfth days. These are the only two days on which she is permitted to engage in intercourse with her husband, as afterward she will experience bleeding for three days during the days of the flow of the zava, and is thereby rendered a greater zava. Accordingly, she requires seven clean days to become pure again, which she will never attain, as she never has more than three clean days. (Sefaria)
No leniencies are mentioned by Rambam or anyone else in this situation.
As far as practical halacha in such a case, many times a rabbi will find that it’s not actual menstrual blood (maybe an internal wound or such), or allow for the woman not to look except the minimal required amount (assuming it’s not flowing like a period), or other such leniencies. However, if the situation is like you mention, or the Gemara mentions, there is no leniency available and she will be forbidden forever like the Gemara says.
Note: the Gemara is discussing the situation for the period in time when they still counted the eleven days of ziva, something we no longer do. Therefore she was able to get two days in after the first time, but not after that.