I buy the 37.5 lb bags of Royal Canin for both of mine. I think it lasts around 3 weeks or so for both dogs, I've never actually kept track of how long it takes them to go through a bag.
Also wanted to state that a lot of dog foods have "Natural" this and that in the name, but that doesn't necessarily mean it is really natural or holistic. The world "Natural" I imagine is a big selling point. The food I get is called "Natural Blend," but that is just the name of the formula. I don't consider any pre-made dogfood to be truly "natural."
You may have been referring to the raw diets some people feed, but I just wanted to slip my opinion in the "natural" food labels in as an aside.
Mom to Tasha, Raven, and Rudy the greyhound
Missing always: Tommy, at the Rainbow Bridge