I find i use around 500mb a month, i think my cap is a gig but i find that hammering the net reduces the battery by faaaar too much! i generally listen to di.fm on the way into work, i have bad reception at work so cant listen to much there. I usually have jabber and skype connected all day and then another round of di.fm on the way home.

My phone can usually be close to dead round 11pm dependant on the signal at work but like i said i only use around 500meg a month.