my buddy has a 20 foot and it works great.
you can prob shorten it by tying it off if you wish.
and how long it lasts depends on how well you take care of it when using and storing it. and how often you use it.
it wont stress the bungee as much, and pulling the 20ft 1/2 way back is a hell of a lot easier then pulling the 10ft all the way back and it gives you about the same amount of speed. i have two 10ft. bungees and one will give me enough speed to hit rails in my flat backyard. if i need more speed or want a longer ride i just attach the two bungee's together.
BCski95could you get enough speed to jump an average size chaine link fence with a ten footer