I got 9+ hours of burn from using anthracite nut coal. I can buy it from Tractor Supply. They are willing to take bulk orders. It comes in a 40 lb bag.
I did a test burn in a vintage cast iron laundry stove and was amazed at the heat output and length of burn time, without any maintenance. I used about 2-3 scoops of coal (big red scoop that I bought from Rural King for bird seed. Probably holds 1 quart, or less). It put out 300°- 400° heat. If someone could incorporate coal use into a rocket mass heater, it would be amazing. No overnight tending of the burn at all.
