Two hours is enough, I think, but it also depends on the number of visitors and the time when you climb the wall.

Actually if you really really want to enjoy and take it the sight on a gorgeous day, the entire morning would be fine. 2 hours is only climbing up and down and taking enjoying the view - this excludes the journey back and forth. When we climb other wild great walls, we stayed up there for 3-4 hours. It took us about 6 hours in total from Beijing into the outskirts. However, if you just want to tell your friends you've been to the Great Wall (but not really enjoying it), I suppose two hours is enough - but that would be a real pity....of course if you join a tour, they just herdled you along...so I never join any group tours...