Good question. I say 30'. Start out with a 12' piece to make your first lanyard. You're going to need 2' to splice with and leave some room in case you make a mistake. I think 10' is a good length for trees in these parts. Then you'll have enough left to splice a second one with that I would make even longer, I find three lanyards to be a good standard. One stays on the belt, one goes up as a secondary in larger trees. And I use a third,(60' if you use this advise) a 30'er to use if I am going to moving around in a difficult canopy. I am not a professional, mind you, just a hobby climber. But I have learned here from these guys.