Five or six hours is WAY too long, even if you're using something as slow as a USB 1.1 connection. I have a 64GB iPad that's nearly full with six or seven pages of apps on it, and my backups don't take any more than 3-5 minutes at the longest.
I would see if you're using an app that might be forcing iTunes to backup its entire cache - there have been problems with apps like Zinio in the past causing excessively long backups for this reason, although I believe Zinio at least specifically fixed this with a software update a couple of weeks ago.
Another possibility is that your existing backup in iTunes has become corrupted somehow. Since iTunes normally does incremental backups to save time, a corrupt existing backup can cause it to back up more than it needs to, or to take more time working through your existing backup. I'd suggest deleting that backup outright (you can do this via the Devices tab in your iTunes Preferences), and then trying to backup again from scratch to see if that's any faster.