I sent my my class legion to support. When will I get it back?

You have to recall them.

If your legion is reinforcing another player then you will get it back when either you recall them yourself, or if the other players sends them home. Or if the other player is attacked an you are defeated.

If they are stationed on a tile or in a building then you can recall them or they will come back if they are attacked and defeated.

If you are still looking for help with this game we have more questions and answers for you to check.