This depends on the type of food and the level of activity of your dog.
If you feed a premium quality kibble complete food then you will need to feed less than say a flaky complete food, or tinned meat and biscuits. Follow the instructions on the bag.
Basically however you should be able to feel your dogs ribs and see a bit of a waistline just behind the tummy where it tucks up. If there is a straight line from front to rear then she probably needs to lose a few pounds. The best way is to cut her food a little and see if she loses any, and increase the exercise gradually. Also, if she is very overweight, you will no doubt have people mentioning the fact to you on a regular basis, and even if you find this offensive, maybe they are right and you just haven't noticed how fat she is! On the other hand, if people keep saying how skinny she is, perhaps you need to feed her up! Being a Lab though, my money is on 'over' rather than 'under'!