I've watched them all at one sitting on 2 seperate occasions...
The first being on the cinematic release of ROTK. The cinema showed the extended versions of the first two and then showed ROTK. They timed it so that ROTK would start in the first few minutes after midnight.
The second time being last Xmas, when I watched all three extened back-to-back. And I'm in no hurry to do it again anytime soon
