It depends on the size of the breed.

Large breed dogs need to be switched to adult food sooner to limit the protein and calcium intake. You want to prevent bones from growing too fast, thus causing joint problems often associated with larger dogs. Most breeders of large dogs - especially those with a breed history of joint problems (such as Great Danes, GSD's and Labs) - will stop feeding puppy food as early as 3 months.

Smaller breeds can stay on puppy food for longer, although you do want to think about the activity level of the pup. Puppy food is higher in calories, so if your pup is more of the couch potato they should be switched earlier than the pup you are training in agility or other sports.

Personally I think the dog food companies would like dogs to stay on puppy food longer because they charge more for it. I think most dogs can switch to adult food at 6 months and if they have high activity levels, you can supplement their kibble with other means.

Here is a good article on why it is better to switch to adult food:
http://www.golden-retriever.com/pupp...adult_food.htm

Hope this helps.