Puppy food is rich in calories and supplements that help your baby grow. If you don't transition him to adult dog food, though, all those extra calories will just pack on the pounds. The transition age varies, but with any dog, it doesn't take more than a week to switch.
Learn when your breed of dog typically reaches physical maturity. There are no rules that apply to all dogs regarding when to switch over, as different breeds physically mature at different ages. For example, a small dog like a pug may be ready to switch to adult food when she is only 1 year old, while a big dog like a labrador may not be fully grown until she is twice that age.
Monitor your dog's eating habits as she approaches maturity. Some dogs will self-limit their food intake when they come of age, as they know they don't need the calories. While this is a sure sign to transition to adult food, it's a sign not all dog owners get -- if you're waiting for the dog to cut herself off from puppy food, you could end up waiting forever.
Mix your dog's puppy food with adult food to make the transition. Switching cold turkey can cause digestive problems, and your dog may resist the change. Instead, on the first day of the transition, give her a mix of 3/4 puppy food and 1/4 dog food. The next day, give a half-and-half mix, and on the third day, give a mix of 3/4 dog food and 1/4 puppy food. On day four, she should start eating adult dog food on its own.