The answer to your question depends on how high you mean to pitch the tarp and whether you need the cords to run to the ground. I think you will need all 50ft. That's assuming an A-frame pitch which requires at least 6 lines: 2 lines that go from the center grommets of the tarp to two trees and 4 lines for the corners. I think 8 feet per line is about what I would want if I wanted to just use it for a ceiling. With that much line you can put the ridgeline at or near head height (so you can stand up) and have the sides about 3 or 4 feet off the ground so you can get a good view while you are eating.

It's possible you could use less if you happen to be in a place where the trees are miraculously spaced so you can tie all the lines off to tree rather than to stakes on the ground. But don't count on it.

It sounds like you have the same Equinox 8x10 tarp (also known as the campmor tarp) that I have. If so then that tarp is designed to be tied to a ridgeline that you string between trees. There are grosgrain ribbons for tying to the ridgeline. I've tried that also but that method uses a lot more line and so 50ft wouldn't even suffice unless you pitch very close to the ground.