The practice of always having a full bowl of dog food out for your dog to eat when he sees fit, is really not recommended at all. Dogs that free-feed tend more to obesity than dogs with scheduled feeding times since it is almost impossible to monitor how much your dog is actually eating each day.
The Guidelines on the Dog Food Bag
Every dog food bag should come with a set of feeding guidelines specific to that dog food. These guidelines are usually based on the weight of your dog (ie: one cup for every 50 pounds of dog, twice daily), and are a rough estimate of how much your dog needs to meet his nutritional needs.
Cooking (or not cooking for raw) for your dogs is one of the best ways to feed them, but certainly not easy. And calculating exactly how much your dog should eat in one meal is even harder, since he won't have the willpower to walk away and leave some of it behind when he's full. Thankfully, Raw4Dogs has a handy calculator to help you figure out how much your dog should be eating.