For the most part, labradors have bottomless stomachs and will rally to beg or eat, even after consuming a full meal. So don't take their word for it. The ultimate answer for a healthy adult dog at their ideal weight will be in answering this question: "Is my dog maintaining a stable weight?" If the answer is YES, you are totally feeding enough. If this is the case but your lab is still begging intensely, it could be due to a number of things like not feeling full (this is a volume issue rather than a calorie issue) or some health issues which means that a quick vet check-up isn't a bad idea. If they are losing weight, food might be the issue but other things could also be going on (we and you/your vet should talk about it) and if they are gaining weight, ditto.