I have a 1GB/month fair use cap on 3 in the UK, but it seems that I only use about 25MB/week on average. This is unconstrained usage, but I simply do not perform bulk downloads on the device over 3G (podcasts, video, etc.), and it is hooked up to wifi outside working hours and normally at weekends.

Last week I went abroad and purchased a local SIM and 50MB data topup. I'd used about 28MB of that when I left a week later. Again this was supplemented by wifi, but I did make some Skype calls over 3G, and used GPS/mapping quite a bit.

Obviously I could consume more data, but this seems so typical for me that I thought I'd mention it.