A six month old Lab should be eating food for large breed puppies to slow the bone growth rate. He should be fed at least twice a day. Check the back of the food bag and see the recommendations for the breed and weight of the dog. Then break that amount up into two feedings. I would suggest that the dog be feed this way throughout life as this breeed can get bloat which is a twisting of the intestines and life threatening. I would also raise the food dish to chin height for better digestion, not allow the dog to drink right after eating and do not exercise the dog after eating for at least a half hour.
A general rule of thumb for feeding is whatever the dog consumes in ten to twenty minutes is enough and leftovers should be picked up.
Here are more feeding tips.