10000kb
This depends on what kind of websites you are visiting. If you're only going to simple HTML or PHP sites, at about 100kb each, then 3gb would last about a month. If you're going to media-rich pages at 10mb (or 10000kb) each, with mp3's integrated (such as myspace), of viewing a lot of photos, you're looking at 2 weeks. If you're constantly streaming youtube videos, or using other flash apps like yoville, then probably a day or 2.