The traffic highly depends on the number of players in the game, the players' actions per minute and the game length.
If you are watching a FFA game which lasts longer than 40 minutes, the client may take more than 1 MB traffic. An 1on1 (20 minutes) generates about 500 kb.
These are values for the orientation, the traffic differs from game to game and it is about equivalent to playing/observing the game yourself.