Last night was in the mid 40s here. I used (2 or 3) 6" x 22" logs (approx) overnight. On average, I would guess that in the 30's, its about a firebox of 22" wood in 24 hours, give or take a bit. We're heating 4,000 sf, to about 73 degrees with a 3-zone hydronic air system. Not the most efficient system in the world.