When did Saint Michael live?

Michael is an archangel. He was created by God as an archangel. He has never lived a human life.
St. Michael, if by him you mean the Archangel Michael, was created by God at the beginning of creation before mankind was made. He has lived since then. He lives today and shall live for all eternity in the Christian view. He is a pure spirit who is said to protect us from the devil. Now if you mean another St. Michael, please ask the question again and specifiy WHICH St. Michael as there are many human St. Michael's. This one, however, is an angel.