I would say based on your pups weight and age is when you should change their diet gradually and of course always speak to your vet first. With Armstrong it was more of a digestion issue so its hard for me to give you advice on how long to feed him puppy food but i want to say i eventually switched to adult food at 8 months to a year. It was definitely longer than 6 months. He is also on just a regular adult formula which does not preference large or medium breeds just adult. But i would recommend talking with your vet because they know your pups history and will steer you the right way.