Yeh my definition of burn time is from reload to reload. Typically I will not reload untill I'm left with a shovel load or two of coals. What degrees to you wait for untill you close the primary air down? Any way you could upload a video of your stove burning?My stove came with secondary air, so I really can't help you there. I haven't even operated a stove without secondary air in the last 18 years.
It really depends on what your definition of a 10-12hr burn is, with my stove after 10hrs I really only have a small shovel scoop of glowing coals left in and amongst the ashes. The bulk of the heat with mine is already released by the 6 hr mark. A lot of stoves with secondary air the size of yours are only quoting 10hr burn times as well, 12 hrs is a pretty big stretch for a non cat stove. A big reason for this is emissions, you need to burn enough fuel to keep the firebox hot enough to continue with the secondary burn. A long smoldering fire isn't good for creosote buildup, efficiency, or particulate emissions. The only way I know how to reliably stretch burn times to 12hrs or more is to buy a catalytic stove built specifically for that purpose.