at most... it would return something like 120 true damage per hit... life steal still negates this, but currently having it as magic damage is pointless, it reduces an already super low kickback number on an item that is outclassed in almost every way by other items
People can argue all day long that malphite and rammus make godly use of thornmail, but realistically they dont... at all
Youre only downvoting because god forbid the game has balance around its items, you would rather steam roll everything and everyone with zero competition to stroke that online ego... sad really