Don't know the actual distance for sure, but I maxed-out my suspension on both sides one time on the AT. It was the only 'good' spot in sight and I didn't want to leave the group I was traveling with at the time. I had just enough of a tail on my webbing to make the backup knot on both sides and the ridgeline on my ElDorado was set at 101", with 11' of webbing on each side. The trees were fairly small in diameter, so take your best guess on the actual span. I had to use my poles to push the webbing up the trees and had serious drop when I got in the hammock. One of my least comfortable nights in a hammock, actually.
I've never 'settled' for a hang site since. Give me 15' between the trees and I'm a happy Cannibal.