Do not even attempt to breed your lab before making sure she doesn't have hip dysplasia! After the age of 2, any signs of the deformity will show up on an x-ray, and only if she is healthy (no signs of dysplasia) should you breed her. Your vet should give you the same advice; you do not want to pass this debilitating condition to a whole litter of pups!
Unless you are a male Labrador, I'd say 'never'.
Anyday but make sure is your lab has a boy boyfriend
You mate her when she's old enough to take care of puppies, and you have the proper equipment to raise a litter of puppies