From what I know and my experience with 3 OHS's, I had been told to expect 3-4 hours in surgery per valve, if it's done the sternal route. (oh, and also expect 3-4 hours added on during redo's to get thru the scar tissue).
Also, I think it depends on how sick you are going into the surgery. The expert surgeons are excellent in the stitching dept. I'm not sure it matters to the best ones which valve they give you. I know my surgeon didn't back away from whatever I needed or wanted. I found out before my 3rd surgery that the mechanical had been placed inside my deteriorated porcine valve during the 2nd surgery to replace my aortic valve. When I needed my mech replaced after endocarditis, Dr Miller cleaned out the whole area of that 2nd valve, and then moved the new valve higher up on the aorta to avoid that used tissue with the new mech aortic valve and graft. I agree with the others that picking the best surgeon you can is always the only way to go!