I used 853MB last month. I'm with Vodafone UK and for the first time in a year they actually contacted me to say im almost past my 500MB allowance and they will charge £5 for the next 500MB.

I usually leave 3G on all day until I get home (then wifi). 3G is just so the super Non-Push email service can update every 15 mins. Some ligh web browsing and thats it. Multiply that by 30 days and the occasional video fest on youtube when im out and you have >500MB a month.

I was on holiday recently and had only 2G available. In 2 weeks with HEAVY use i just about got to 700MB in 2 weeks. Chat was always on, web browsing done and flickr uploads of original sized images. So with 2G you are in good hands (though the speeds makes you want to cry when browsing the web- though IM/Chat isnt impacted by the slower speed).