I'm in Maine, and the thought of reclearing some of the (now forested) land bisected by thousands of miles of stone walls seems incredibly daunting. Not just because of all the trees, but the stumps and fieldstones as well -- and i'd be using modern machinery!
How long would it have taken someone in 1720 to clear an acre of this rocky, hilly, forested land and get a plow into it?
Maybe not exactly the answer you are looking for but very often there were fields left over by the native American tribes. So the settlers didn't have to do as much work as it may seem. This is touched on in 1491.
Sub-Question. What did this process look like? Given that wood is used for fuel and building materials, was the creation of new farming plots done whole sales, was it a gradual processes of expansion, or some combination of the two?
In "Rough Stone Rolling", a biography of Joseph Smith Jr, historian Richard Lyman Bushman says that "a man working alone might clear ten acres the first year [of cultivation]" (pg 33).
Of course, this refers to the 1810's in Palmyra, New York. Bushman later says that Joseph Smith Sr, with his three young adult sons, cleared 30 acres in a year. I'm not sure how much the technology would have changed, but the Smiths would have been using axes, fire, shovels, plows that were probably broadly similar to those used in the 1720's.