I believe it depends on which packages you already have installed on your current version. When you do a distribution upgrade, Ubuntu will download the packages of any software that has since been updated to a newer version in the Ubuntu repositories. For example, 10.10 came with Firefox 3.6 and Open Office by default, and if you didn't manually the latest versions straight from the PPA's, then an upgrade to 11.04 will download the new packages for Firefox 4 and Libre Office (which comes as the default office suite in Natty). This goes for any other updated software that has made it into Ubuntu's repositories.
That said, if you more or less left Ubuntu with its defaults or substituted some alternatives, then your upgrade shouldn't be that large, and I wouldn't expect it to take too much of your monthly quota. However, if you installed many apps that are quite active (i.e., new stables get released frequently), then expect it to take a while.