I would say that downloads (eg music tracks) plus YouTube (and any other websites which teens use to watch video clips), plus iPlayer, would likely push up usage.
One 30 minute TV show runs to 300 MB via iPlayer desktop (I download overnight, because ISP do not count any traffic from 00:00 to 08:00) - so yes, the how long is a piece of string answer comes into it.'
One thing you could ask is how easy is it to check the amount used via the network's website. I've been using a Three mobile dongle and can use well over 100 MB by listening to radio for part of the day... but they allow 15,360 MB a month so it's quite generous.
Before rushing to get the Blackberry (good for e-mail but gets some negative comments about web browsing) you might want to search MN for 'Blackberry' in geeky_stuff or one of the products topics (to exclude food!) and see whether this would be suitable.
I don't know what cost you are considering for the monthly contract but if DD has friends using Skype and you have coverage from Three locally then a Three mobile may be worth considering (can handle Skype and different models will offer web browsing etc).
Probably worth checking on www.GSMarena.com before you get too close to buying to check if a mobile supports wi-fi as plenty do not (while still allowing internet access). Having wi-fi means you can use the net from your home network (or at a friend's house on a sleepover) without using up the monthly allowance, but you do need to know how to control access (ie disable '3G' on the phone/disable the phone function).