The stock can be any length you want it to be as long as the total length of the rifle is 26" or greater, with a barrel 16" or greater. On an AR the buffer tube would be the limiting factor. However, if the rifle is a post-ban (manufactured after Sept 1994) then the stock cannot be adjustable/collaspible unless you are willing to get rid of another one of the "evil" features listed below. As I recall the AWB says that you can have only TWO of the following on a post-ban rifle: Pistol Grip, Detachable magazine, Bayo Lug, Flash Suppressor, or Collaspible Stock.
So what it boils down to on a normal post-ban rifle, is you would have to either get rid of the pistol grip or the detachable mags in order to install a collasible stock. So, lets say you found a way to get rid of the pistol grip and still have a functional rifle. Then I believe you could add a collaspible stock. However, the total length of the rifle with stock extended would still have to be 26."
There are two laws that govern this, the 1994 AWB with says you can only have two "evil" features and the 1934 NFA with dictates rifle and barrel length, among other things.
Something you may wish to consider: Leave it alone until we know what will happen with the AWB. If it sunsets then you can go ahead and add a collaspible stock, but the length rules will still apply.