I was trying to work that out.
Trust me, not mathematically !!
Given the terrain it could be to simply clear a river gorge, in which case a hundred yardmetres could be enough
Guesstimate would be, to make it worth while, it must be a minimum of a quarter of a mile but I would expect a comfortable half mile, 1km.
Perhaps the number nerds could work it out for us mere mortals.
Be happy.