This depends on the activity level of the dog, the size of the dog and the type of food. If you feed a good quality complete dry food, then you would need to feed less than say if you were feeding tinned meat and biscuits.
Try cutting down her food by a third, and see if that has any effect after a couple of weeks. Gradually increase her exercise at the same time. Keep a note of her weight and if she is not losing any cut it down to half the original amount. Split her feeds up into two, so she has something to look forward to and isn't going so long without any.
Don't be fooled into giving her more because she is 'hungry'. She is a lab and labs always try to tell you they are hungry, when in fact they are purely greedy!
If you are feeding a tinned meat and biscuits diet, or household scraps, then consider feeding a complete food, and follow carefully the instructions on the bag. Again don't be tempted to give her more because it doesn't look much - they digest much more of the food so need to eat less. The other advantage is that they poo less! Many of the better quality foods do a 'light' version. I would recommend Eukanuba, Purina, Skinners who all do diet feeds.