I use a 3 Mobile dongle with a monthly limit of 15GB and I do hours of surfing every day with some viewing of streamed video (other than what you might come generally across on websites e.g. BBC) together with downloading programs of several 10s of MB every couple of days or so, and my monthly usage is 9-12 GB per month.
Whilst the penalty for going over the 15GB limit in my contract is very expensive I haven't yet found that I've needed to curtail my internet activity as the month ends approach.
If you haven't already done so, use a program that records your internet access to keep a check on how much you are using - the one I use is Networx
http://www.softperfect.com/products/networx/ which I have found good enough for my purposes but there are various others available.