It is certainly not a natural state of affairs for a dog to have access to food all day and problems can arise from it. It has a very significant effect on the dog's self image, for example. The very fact of open access to food is a clear indicator to the dog that it is seen by other pack members as of much higher status than them. It can create immense problems, as Dixieland Dancer pointed out, if another animal is brought into the family. A dog that never has an empty stomach is much more likely to use food as a manipulating tool - once it finds out that you react badly when it doesn't eat it also finds a way of getting attention. It is very difficult to tell if this kind of dog is off it's food too.

If you want to change the habit start off with very tiny meals of four or five pieces. Leave it down for 10 minutes and remove it. Try to up the activity level of the dog to stimulate it's appetite and feed four times a day until meals are eaten fairly quickly.

Feeding guides on dog food bags and tins are very useful for a starting point. Be careful not to stick to this exactly - seasonal changes have an effect on weight as do health, age, breeding condition etc. etc. etc. (Paddy goes from 2 measures all the way up to 3 and half during the course of the year.)
Keep an eye on the weight of each individual and adjust the amount of food gradually to compensate for any loss or gain. Give at least ten days before changing the amount again as too fast a gain or loss can be damaging.

As for Labs...I know just what you mean - stomachs on legs!! If the dog is looking the right wieght and healthy then don't worry what people say - it's not just Labs!