We have four dogs, but that's not really relevant as they all eat different things.
It takes Winston a little over 5 weeks to go through a 40lb bag of Kirkland Lamb & Rice. He gets 2 1/4 cups per day. He's a large dog (75lbs), but very low energy, so he doesn't need much.
When she was being fed NOW, it took Keira 2 weeks to go through a 25lb (or was it 30?) bag. She was eating 6 cups a day. Now she's on California Natural chicken & rice, which is a 35lb bag I think, so it'll probably last about a month or a little more as she only has to eat 4 and a bit cups a day of it.
The Tollers were on raw, but due to money situations and Dance's intolerance to so many different things, they're on California Natural Herring & Sweet Potato for the time being (they'll be switched back to raw in a month or two probably). I bought a 20lb bag with both eating just under 2 cups per day. Not sure how long it'll last, but hopefully close to a month.