I think it depends what you mean by 'idly'. If you're literally not doing anything (i.e. no web pages open at all), then, yeah, probably a long time, but as soon as you start opening/closing web pages, it starts eating up data, and even computer programs can eat some (e.g. anti-virus program automatically checking the net for its latest updates). There's probably an application you can download to track it. - When I was in the Phils, I used to use Globe's Wifi internet, and its router allowed me to track my data usage - Be warned of things like YouTube videos! Even though they're streaming vids, they still can easily eat up tens or even hundreds of MB per video.