When I had mine done, I was told 8-10hrs by 3 different shops that do it regularly. If there is a build going in it then I think it was 12-14hrs.
That is what they were charging at their shop rate. Now if they put 2 techs on the job they could shorten the total time the machine is down, but you would still have that many man hours into the job.
I paid $1000+fuild/filters/gasket for the R&R of the allison and a down pipe on my 08 4x4, It went in at 8am and out the next day at 2pm. I think they said it took a little under an hour for the down pipe with the tranny out. I went under the truck with the tranny out and the down pipe still looked a little trapped in place.
Given how heavy these transmissions are, I was glad I did not do it myself on my back without a lift.
My buddy who runs a tranny shop in my area quotes me 3-4 hours to remove, replace rear extension housing and re-install. As for ten hours, I have that time or less into doing the job 100% on my own, garage floor on stands with no tranny jack. Any experienced shop can do it easily in 4 hours max especially if they've done it before, however if their computer calls for 10 hours, that's what they'll charge... welcome to flat rate my friend lol