You get 80 tons of wood from an acre of land after 20 years of growth, or 4 tons a year. A household probably uses about 2 to 4 tons a year, less if they're in a warmer climate. That's half an acre to an acre per house.

There are potential problems here: water table and soil depletion, pollution in built-up areas including acid rain, overuse of wood in crowded areas destroying habitats for animals, and probably more. Nevertheless, all these problems only apply in certain circumstances, and rural use of firewood for heating is likely not as big a problem as you think it is. Obviously in the city things are different, but neither the author nor letrowekwel were talking about cities.