Depends on the versions of Bowe and Moorer that show up, but I think that most times, Riddick gets a knockout after some rough rounds. I know that Michael's southpaw jab and heavy hands would be tough for him.
I'm thinking somewhere around 9 or more. Hide took him 6 and Biggs took him 8, so I figure Moorer should last more than them.
Bowe didn't come out as fast as Cooper, and the peak version of Riddick we're talking about isn't going to be trying to knock out Moorer unless he sees an opportunity. I think it's mostly gonna be a matter of working behind the jab for both men. On top of that, Michael had very underrated recuperative powers pre-Foreman, he came back from nearly getting knocked out in the first round against Bert to war on and knock him out.
Cooper’s no Bowe, to say the least. I can accept a later stoppage though. I do think Bowe would go for the knockout at some point, however. Moorer could be competitive for a while.