I'm on a 50GB a month contract, and occasionally use 26-28 GB. That's with the kids constantly watching the iPlayer and me streaming Planet Rock plus endless emails, facewhatsit etc.
We get an email when we go over 50%, so long as it's in the back half of the month I don't worry about it. Interestingly, until April our allowance was only 25GB/month, then I was getting a bit twitchy!