If you are on Windows 7/8, go to Control Panel > Network and Sharing Centre and it will show your active network connections. Next to your dongle it will have the type of connection and the name in blue. Click this and it will bring up how much data has been sent and received over this connection since it connected.
Note that it will not count it if you disconnect/reconnect, but with that said most mobile carriers will give you your online data usage, so you can see the total from there. I know EE (So therefore Orange/T Mobile as well) and Vodafone do.
You will find that it will use surprisingly little. My gf can only get mobile data and the 2GB a month she gets lasts her just fine (unless we Skype a lot or she downloads something of course). She found too that Facebook, even if we are just talking on it and she is doing nothing else, uses more data than most games will do.
Seeing as you will only be using it for a short time each day, I would not worry about going over if you even just got 1 or 2GB data.