There is no set time line for a dog to when he's ready for a collar. A lot of it depends on what kind of dog are trying to develop also. For the hunter that likes gundogs that hang in close you may never require a collar. We run a lot of field trial dogs it's a whole different animal. My good training friend has always told me when your dog is figured out that he doesn't need to listen he's ready for a collar. It doesn't necessarily matter the age of the dog it's just when he comes to the realization that he can do what he wants then you need to put a collar on him. If you do your yard work properly the collar comes in handy as a reinforcement tool. The collar is not for teaching commands its for reinforcing commands the dog knows and chooses to disobey. You cant go by hard and fast rules and generalizations about when to use a collar only you and your dog know when he's ready. When you feel like your dog is confident in his commands and he's choosing not to obey them then it might be time to go to the next level which includes a collar. Yard working a young dog is the key though you can teach a dog just about everything you want to know what the checkcord. Good luck with your pup don't be in a hurry with him remember 1 thing he has the rest of his life to be a broke dog he's only going to be a puppy for short time. Enjoy it now while he's young enough that he can't disappoint you