I'm currently on my 7th hour straight. Will be playing the game all tonight and most of tomorrow as well. Only taken about a 15 minute break so my Xbox could cool off.
I don't even like playing it all the time. Playing sessions usually last 2-3 hours - if I play it any longer I feel like I'm rushing things and as a result missing out on the overall experience. (Per example, I get impatient and start using fast travel even though I promised myself I would limit the use of it)