Forget it. No answer will be right. You might start with getting your VO2 measured and determine the percentage of carbs you eat before your dive. Do a stress test and find your heart rate and VO2 correlation. And then bring a submersible heart rate monitor to find your exertion level. Some divers with very little muscle and low VO2's probably could do 10-12 hours going up and down an anchor line on their 15.5s. Do you get my point? Since you have a 15.5, you obviously can afford changing your scrubber every so often.
So here is some info: I had what felt like a minor breakthrough at 4 hours with 4-8 Sodasorb in the Galapagos, but I was in 59 F water in a strong current at 150 feet towards the end. Since I went to primarily Sodasorb 6-12 or Sofnolime 8-12, the most I have ever done is six hours. Observed no more than 1/3rd of the scrubber with purple color immediately after the dive. Since 90% of my dives are solo, I just do not feel relaxed if I do not have a fresh scrubber. There is something to be said for knowing that you could stay down all day if such a situation required it. Announce to your pals that you have a kick ass military rebreather and only have to change your scrubber every month under water. But then privately change it every six hours.
On the other hand, you might want to try leaving your used scrubber unwrapped on the bookshelf for two weeks like I did and experience what it is really like to have a break through. Just once. Nothing like raw experience to hone your diving style.
