Dog food serving sized

Question
How much dry dog food should my 3 year old lab have to eat per day?

Answer
Nobody can tell you that.  Use the method illustrated in this link to evaluate your dog, http://www.puppychow.com/products/popup_body_condition.aspx  You then adjust the dog's chow and exercise to reach the ideal body condition.  Labs are notorious for gulping down more than is good for them and looking for more.  Unfortunately, overweight dogs develop joint problems earlier and live shorter lives.  If you are not sure about your judgment, have the vet confirm it next visit.  

I have had Labs I was feeding anywhere from 3 1/2 cups to 6 cups a day of Pro Plan to keep them lean.  You may need to feed even less to bring his weight down.