Was Ruth converted to Judaism in the Book of Ruth? If yes, when did it happen? By the time she marries Mahlon, or by the time she follows Naomi to Bethlehem, or by the time she marries Boaz, or none of the above?
Ruth's decision to follow the Law of Moses is seen in her vow to follow Naomi's God and be a part of Naomi's people. "But Ruth said: "Entreat me not to leave you, Or to turn back from following after you; For wherever you go, I will go; And wherever you lodge, I will lodge; Your people shall be my people, And your God, my God. Where you die, I will die, And there will I be buried. The LORD do so to me, and more also, If anything but death parts you and me"" (Ruth 1:16-17).
It wasn't called Judaism at this time. That name arose after Assyria destroyed the northern tribes leaving only Judah left.