Completely depends on how you rig it. There are several variations of a continuous ridgeline.
What I do is draw it out on paper and label the length of each segment. Then add up all the segments. Don't forget to add a bit for knots or buries. If you are making a ridgeline to use with more than one tarp, do this with your shortest tarp.
But first, you need to decide the diameter of tree you need to account for. Then the clear distance between trees, which depends upon how tall you are and can reach to hang your hammock.
I use Dehoja's variation of continuous ridgeline with an 11' ling tarp assuming 18" diameter trees and a 20' clear distance between trees. I used 37' of 1.2mm Tech Line from www.DIYGearSupply.com