I always feed a dog or puppy based on how they look. Until your puppy is full grown it's best to keep them on the lean side. That means a clearly discernable waistline and being able to see a couple ribs when they're moving around.
That being said, all dogs develop at their own rate, so there really isn't a set age for you to cut back on his food. But you'll notice when he stops growing fast, so just gradually decrease the amount then.
I could see most of my dog's ribs clearly when she was running, and I was feeding her as much as 6-7 cups when she was growing fast. For her the upward growth really stopped at 9 or 10 months, so I tapered her food gradually back down to 4 cups, which is what she gets now. The actual quantity varies greatly by the type of food you're feeding though, so it's usually best IMO to go with the amountbsuggested on the package, then adjust accordingly to keep your dog a bit lean.
It's not the size of the dog in the fight, it's the size of the fight in the dog. Mark Twain