Pischei Teshuva (Even Ha-Ezer 1:19): Until the end of the 5th millennium (1240) - these are the words of the Maharik who cites Rashba that he had heard that Rabbeinu Gershom had not made his decree to be in force after the 5th millennium... Look at Mishkenos Yaakov (#1) who writes that it seems to him that the reason for the rumor - even though normally a decree is forever until a beis din arises that is greater in wisdom and number as is well known – is because all rabbinic decrees need to be a protective fence for Torah laws. This is expressed by Ramban (Eschanon) that one should not create any matter or decree except that which is preventative fence for the Torah. The only exception being that a beis din has the power to make temporary decrees for the needs of the time and generation in order to correct temporary aberrations. Those matters which are for the needs of the time are not made permanent decrees. According this understanding, we can suggest that the ones who said there is a time limitation to the Cherem thought that Rabbeinu Gershom did not make this decree as a protective fence for a halacha. That is because there is absolutely no Torah prohibition which would justify this fence for everyone. Thus those who make this claim feel that the decree must have been made for some transient problem of those times and that is why it was only for a limited time and not permanent and it wasn’t made in connection with a Torah law. However this view is not unanimously held and there are those who disagree with it. This issue was raised by a few gedolim cited in a teshuva of the Beis Yosef. Besides this there were also a number of gedolim at the beginning of the 6th millennium such as the Ran, Mordechai Maharam Ruttenburg and Raviah who discussed the Cherem of Rabbeinu Gershom and yet did not mention this leniency at all of it only being in force until the end of the 5th millennium (1240). ... According to all this evidence we must conclude that Rabbeinu Gershom made this decree as a fence to protect Torah prohibitions because of various reasons and therefore this decree must have also been made forever as we find with all decrees that the Sages made. And Maharam Alshakar (#95) writes, Concerning the decree of Rabbeinu Gershom this that it is claimed to be in force only until the beginning of the 6th millennium – but it is already well known that the majority of the Jews in exile never accepted this decree then or now - for example Spain and the West and the entire East. There are places that did accept it even until today and they conduct themselves in accordance with it. Therefore for all those places which accepted it even until today - it is in force as it was during the 5th millennium.