Some time in the Second Age, although the precise year is unknown. The rings haven't been forged yet, which according to Tolkien places it before the year 1500. However, due to various things like Isildur being alive 1700 years before the rings are forged means the official timeline has been done away with.
Sauron hasn't yet been officially introduced into the show in terms of an actual on-screen character taking part in the story. One or two characters are under suspicion of being him but I don't think he's in the show yet.