From what I've learned scouting different areas is that length is dependent upon the types of trees. Around my house, oaks and maples are the primary trees and a longer rope, 80-100' would be the best for a base anchor. Rationale being that these trees have multiple strong limbs up high with many more, smaller limbs below them and trying to fish you rope around that single limb is a PITA.
Conversely, where I hunt it's predominantly "telephone pole" ash trees with few limbs and without strong limbs until to reach the canopy. In these cases, the only option is to girth hitch the trunk using a thin limb to hold the rope in place until you run the canopy anchor up. For this, 40-50' is plenty.