Mark...

How long is usually determined by the size of your cord. I use 30 feet of 1/2" dia surgical tubing with 1/8" wall; this allows me to use it on minifan models and some midi fan models.

The rule for stretching is around 5 times the weight of the plane. For example, if you have a HET-RC FA-18 that weighs 36-44 oz (near 3lbs) , you should set launch tension to 15 lbs. I recommend you use a digital fish scale which is very accurate. I have used the digital and a cheap fish scale and the cheap one read 19lbs when the digital one said 15lbs...just something to keep in mind.

Hope this helped,

Carl