When did Jewish faith cease polygamy?

Answer 1

Polygamy among Ashkenazic Jews didn't cease until about the year 1000 CE, when Rabbi Gershom issued an edict banning polygamy for 1000 years. The edict has since expired, but the ban is still followed, mainly because all Ashkenazic Jews live in places where polygamy is illegal.

Sefardic Jews living in some Arab countries still practiced polygamy until the 1950's. After that time, most had moved to Israel and the United states, where polygamy was not part of the culture. Today, very few elderly Sefardic Jews live in polygamous marriages.