Depends on what food your feeding and how active your lab is. I would start at 2 cups, check your pups body condition - you should be able to feel the ribs easily but not see them and your dog should have a visible tuck - and go up or down from there.
Example: My 3 year old, spayed female, lab mix eats 1.5 cups Orijen six fish and 3/4 cup of The Honest Kitchen dehydrated food a day. She gets approx 1.5 hours of structured exercise (i.e.: on leash walks, jogs, runs, small hikes) a day and about 30 mins of free for all (off leash fetch/running) exercise a day. She weighs 31.5 kgs (finally!) and is slowly getting down to her ideal weight of 28-30 kgs.