It all depends on your rate of work. Obviously if you're a hard hitter you might be able to get through things fast. For me, with TONS of playing every day, it took about 2-3 days. There's a LOT of collect, defeat quests in Avalon; it definitely takes a while. Again though, it is definitely based on a lot of factors.
Avalon can be done in 2 days (I have), if you really don't do much of anything else other than meals, bathroom and a small amount of sleep. I would say for most players it takes closer to a week, playing for several hours a day. If you are limited to just 2-3 hours a day, probably a couple weeks.