jetsam
Minister of Fire
So you typically run the stove on a 3 o'clock setting. That in itself would explain the short burn times. No one that I'm aware of can achieve 20 hour burns at the setting. How tall is the liner? Age of your home? Insulation quality? It could be that your heat loss is quit bad. Some exterior masonry fireplaces suffer badly from heat loss to the outdoors, Roxul insulation around the unit can make a big difference!
3:00 is where I run mine on the coldest days of the year when I need very high heat output, and I suspect it's an even higher setting for OP (15' flue vs 25' flue). On my short flue, I would expect maybe 6-8 hours with no fan on such a high setting, and maybe 4-6 hours with a medium fan. I don't run it that hot with the fan off though; feels like I am heating the flue more than the room at that point.
Even on a short flue with no fan and wet wood, 3:00 is blazin' hot at my house (but I am used to never letting the stove go down to low coals, because I'm still burning suboptimal wood on my second year in the house).