To be honest, I thought I was a "heavy" user, and I thought I took advantage of the "unlimited" data O2 offered back when I got my iPhone 3G in 2008.
Once my contract was up, I moved from O2 because of their 500MB policy. Orange offered a 750MB "Fair Use" policy which meant that if I exceeded it occasionally, I wouldn't get penalised. I thought 750MB was nothing.. and I'd be exceeding it all the time.
Well, my monthly data usage is around 50-150MB. Touching 200MB on very rare occasions. I view videos, browse the web and download apps (that are downloadable over 3G). I'd have thought I'd be using around 30MB a day doing that which would be 840MB a month.
I wish I had gone with Vodafone - they had a 1GB policy, but from what I read, it seemed they would enforce it more than Orange would (with possible penalties for exceeding). Though, I'm never going over 150MB a month these days..
I'd check out how much data you are actually using per month, because you might be surprised by how little you actually use. You may well use over 1GB a month though, but worth finding out your actual data usage.