The Carretera Astral is a very nice route, I loved it. I did it in about two weeks, but I cut into Argentina at Futaleufu. Lots of cyclists do this, because you have to take some ferries to get to Puerto Monte after that.
I did it in 2 weeks, you can read about it starting here:
http://awesomebiketour.tumblr.com/po...o-yungay-chile
Although I agree, more time is better. Also the end at Villa O´Higgins is remote, so getting to an airport from there could take some time.
I did it this last December. It was very wet, and can be very cold. It´s beautifull though. Mostly unpaved, rough roads.
The Volcano region around Villarica is also really nice. You could maybe do some sort of ride around there, and go to the different national parks in the area.