I agree with Jouster... none of us here are going to be able to give you any answer. The only possible answer I can try to give you would be something like "Since it's free, just grab 100 feet of it, and you'll have plenty"
Other than an answer like that, you are the only person who can answer it. Go look at your CP and decide how you want to run your wire. Measure the path, add up the number of connections and add some for extra as mentioned. If your CP isn't built yet, look at a similar sized box, or make a cardboard mock up of the top.
Or just go get 100ft or so.