Put a biner somewhere toward the center of the rope on one end of the hammock with a bowline knot. Now you can quickly hook up one end of the hammock for support while you tie up the other end to the needed length. In other words, make a semi-permenant biner attachement on one end, and leave the other end free for you to tie and adjust as you need.
You can also use the biner on one end and loop around the tree or around a slap strap/ webbing loop and secure back onto the hammock rope by wrapping twice around the biner. The biner will hold this way and you can adjust the tension by sliding the biner back and forth on the rope.