Depends on the fuel and the burn rate. The fastest I can burn one load of 100+ pounds of the good stuff is 16 hours. When running at high burn, I usually back off on the quality of some of the fuel so I can stay on 12 hour cycles. This is wide open, blowers on high. That same load will burn 48 hours at low burn but I don't have enough of the good stuff to waste when I need that little heat.