It takes a while when you add a lot of new apps to the iPhone, or stuff for the apps. For example, loading 400 MB of data into Airsharing caused my next sync to take 45 minutes. But after that it was back to 1 - 2 minutes. Also, if you cancel a backup (by clicking the "x"), you corrupt the backup. And the next time sync the backup will need to start anew, and therefore will take a looong time. So don't click the "x" unless you mean to never backup.
The two annoying things I hope they fix is the fact that the backup corrupts, or at least needs to start from scratch, the time after it is cancelled. Also, if you get a call during the backup it automatically cancels, and then naturally, needs to start from scratch the next time it backs up.