AVG 8 will scan more "deeply" so it's reasonable to expect the scan times to be longer. 6+ hours does sound excessive to me but if you've got 36GB of data, it just might need a lot of time to scan the system. Additionally, you using the system while it scans might slow the scan down a bit.
What speed CPU does your computer have? Can you schedule the scan to run over night, while you're not using the system? On the machines I've installed AVG on I schedule the scan to occur at 1 or 2am and I ask them to leave their machine on at least one night a week. This way, AVG will scan while they are not using the system and they won't want to terminate it due to system performance degredation from "spinning" the hard drive.
Another thing you can do is look at the scanning settings and maybe have it not scan compressed files or things like that. If you can schedule the scan to run when you're not using the system, I would recommend that.
Peace...