It really depends how cold it is outside. If its mild brush will last maybe half a day at most. But when its cold I will burn through brush in a matter of hours.

Usually when it gets really cold I burn larger logs and less brush. But when its warmer I go through much of the brush I have.

Yes there is build up, but I go behind the furnace about once a month and clean it all out using a stick.

They recommend non-green wood, though honestly I do burn it sometimes. Any kind of wood is fine as long as its not pressure treated, or processed like plywood or particle board. Only natural wood from trees.