Let's discuss this a bit.
I think that "longer is better" is mostly a misconception, especially when larger models are involved. It's like saying "more BP is better" or "more thrust is better."
--Longer chock cords do not "give the parts time to slow down," except for very light or very draggy models, or degenerate cases involving extremely long shock cords and rockets deploying well before apogee. After apogee, all parts of the rocket are always accelerating due to gravity. A really long shock cord just lets the part without the parachute attached accelerate (downward) to a higher speed before being brought to an abrupt stop at the end of the shock cord. This is not A Good Thing.
Further, for practical purposes, the relative velocity among the parts that is imparted by the ejection charge does not change appreciably before the end of the shock cord is reached (unless the cord is hundreds of feet long
. The aerodynamic drag just isn't big enough.
--Longer shock cords can enable the parachute, if it is attached near the nose cone, to fully open and then be hit by the fin can, following in trail. This happens more often than you might think!
--Longer shock cords _significantly_ increase the odds that your rocket will end up in any downwind trees and powerlines you may have. Think of it as making the target (the tree) much bigger with respect to the net (the shockcord).
So, how long should the shock cord be? It usually only needs to be long enough to enable the parachute to get out and get opened. If you attach the parachute to the fin can, and not the nose cone, the shock cord can be fairly short--it has to be longer than the rocket, to keep the nose cone from clanging against it during descent. If the nose cone is quite heavy (e.g., if it is full of weight), you can use a longer shock cord to let the nose cone land first, and give the fin can time to slow down before it lands.
If the ejection charge is forceful enough to cause the nose cone to reach the end of the shock cord and bounce back, there is an argument for increasing the length of the shock cord to reduce the chance that the parts will collide on the bounceback--but there's a better argument for decreasing the ejection charge!
It's far more important that the shock cord be STRONG enough. Deployment forces, especially during early or late ejections, can be inconceivably high-- a ten pound rocket can generate 1000 pounds of force fairly easily--so everything you do to manage those forces and size the shock cord to handle what you might not be able to control will save you rockets in the long run.
That's my opinion, FWIW, but I'd be happy to hear other points of view!
--tc