Just took an hour long walk through my land with a professional forester. He says that if we log 20 cords per year for heating the house, the forest will produce wood faster than we can harvest it. Squares with what I had thought, from reading, but good to have confirmed.
10-20 acres. The 1 cord/acre/year is a rule of thumb. Reality varies up or down based on (a) local soil / climate / rain / species mix, (b) average age of trees (by analogy, teenagers have growth spurts, and so grow more inches/yr at 17 than at 39). Bc of a and b, I get 2c/a/y