Will vary a lot depending on what you plan to use it for.
As a rough rule of thumb I reckon 500MB should be about right for most people who just surf the web, email, social networks, few random apps and maybe the odd video clip here and there. This is roughly in line with my own usage, but I use WiFi at home (no 3G signal or sometimes even GPRS signal) and tend to save any sizeable downloads for wifi.
My last few months were roughly as follows:
290
170
480
45 (was overseas for a week)
68
Besides the obvious of downloading stuff (easy to measure/predict) Video and tethering is probably the real killer, the month I used 480MB I was working away for a few days and tethered my laptop. Problem with tethering is you don't usually get served mobile sites (so much more image data etc), depending on application/system settings you might get some random updates etc downloading in the background.
Realistically only you will know, personally assuming you have a decent level of wifi access (at home at least) I'd start off with 500MB and see how you go.