Yes, such a wide range in burn times even in the same stove! I usually rate max burn time as down to the last red-hot coals that can be loaded with small splits and take off without needing kindling. For the last few years I've been burning only ash during the coldest part of winter. On zero degree days I may only get 6hr burns running it 600+ stovetop. I have however done a load of hand picked all square large splits of ash and packed them in tightly, turning down the air as soon as in the cat active range and scorched good and got almost 24 hrs (I think it was 22 but can't remember without my notes handy). With compressed wood bricks I've been able to pack in 84 pounds of wood and in full cat mode (black firebox, no visible flames) had a burn of almost 50 hours with a pile of red hot coals remaining and stainless steel center burner on top hoovering around 300F at the end after 2 days. Then again I just burned a 2/3rd load of spruce and got 3-4 hours lol. So you could say it has a burn time of half a day +/-400%
I just realized I am not sure if I have ever heard of the PH being able to burn in 'full cat' mode with no visible fire.
The really long burns need a good working cat. This year mine is not working well, seems to expire much before I thought it would. Still trying to find a large enough pot to do a vinegar boil to bring it back to life.