user wroteis it any different from online gaming on PC?:P I doubt it is, and on the pc you'd be using up like a constant 128 bit stream, so maybe 40 MB per hour
It's not the same for all games. It depends on the number of players. For server-based games, DL bandwidth is proportional (nx where n = # players), UL bandwidth is constant (x). For P2P games, both DL and UL bandwidth are proportional since you would be uploading x to every player (nx) and downloading nx.
Thus, total bandwidth for server-based games (like BF3) is (n+1)x, while total bandwidth for P2P-based games is 2nx. x is different for each game but largely similar for games of the same genre. For a first-person shooter, x represents movement / position, action (shooting, crouch, etc...). The smoother a player's movement is apparent to others, then the more bandwidth is required for movement alone. Actions such as shooting, crouching, etc...are small in size. In strategy games, information is needed for each unit that is performing an action (moving, attacking, etc...). In these games, it is incredibly difficult to gauge bandwidth consumption since it would be variable for each game. It is, however, possible to do so if you have a video replay of the game you just played, along with a graph illustrating bandwidth consumption based on time.
The network code for a game is the easiest method to know how much a game consumes for what you are doing in it. Observation would get us close, but it would need a recorded scenario. It's not magic.