With a general rule of thumb, it is about 40 - 100 mB for about an hour of online gaming. Play on the local servers though so at least if you are capped, you can still play. So, you can play for about 1 - 2 hours every day and not reach a cap.
Depends on the game you are playing --> Type of BF1942 Game and also your network setting under BF1942, Number of players etc. All server factors add up. Also possibly your server settings.
I can understand your frustration Dominic, I have to restrain myself and set bandwidth limiters so i don't kill all my bandwidth on games. I use a program called bandwidth meter, to tell me how much my games/internet are chowing. slip me a PM with ur email add and i will hook u up with a hmm how to phrase it... a licensed version of the program.
I am using an app called NetLimiter. Brilliant app, allows me to set my speeds per Bandwidth chewing program. Only thing is activate the program BEFORE you start downloading. Starting it in the middle of the download tends to corrupt the file