I am with t-mobile uk, i used to have 2 contracts with them before i had wifi and both contracts had web n walk plus (3gb per month) and i went over my limit on both sims a lot, yes i used over 6gb of data lol, and that was with a nokia n96, so be careful now we have a full web browser it will eat a lot more data.

*note i used to upload/download a lot of files per month, web browsing alone would not amount to this much data.