I'm on Orange, my last bill showed a data usage of 211mb. This covers facebook and twitter access daily, news feeds, gmail, light google maps usage, the weather app, occasional MSN usage, light browsing (probably 20mins a day, mostly mobile versions of websites), the odd youtube clip, very rare usage of beebPlayer and probably 2 apps downloaded over 3G. There might be a spotify track or two included in that.
It's youtube, beebPlayer and streaming audio that will kill it. For things like that or app downloads, you're best off using wifi - most of the time if I'm streaming video I'm in a coffee shop, and they all have wifi these days.