Hmmm...it probably depends on the breed of dog, and also what you'd define as "puppy stage"! I know that some breeds like Labs tend to mature slowly; I assume the bigger the breed, the longer it takes to grow out of spastic behavior, destructive activity, and a short attention span (I guess what most people would call the puppy stage).

I was surprised at how quickly Eli's puppy stage passed--he was a pain in the butt until about 8 months old, and since then he's been a joy!