From an old forestry book ~1950's, which unfortunately has flown the coop, it was suggested to never remove more than 1/3 of the crown at a time. It was definitely worth pruning the first 16', a toss up on the next 16' and a loss to prune the next 16'. They also said it begins to pay off 40 years after pruning in quantity of clear lumber produced. That would vary greatly if timbers are desired though. Its a pretty nice way to spend a few hours in the woods.
