The answer to this depends on various factors:
1. over what distance?
2. the lab's weight - i.e. is the lab in good proper weight or is the lab overweight?
3. what shape the lab is in? (is the lab exercised regularly and in great shape or is the lab a couch potato with flabby muscles?
4. the lab's structure - i.e. does the build (or structure) of the dog make it easier for the dog to run fast or harder for the dog to run fast (if the dog is more angulated in his rear than his front, then his rear legs will interfere with his front legs and slow the dog down).
5. the lab's motivation - i.e. is the dog motivated to run fast? Does the dog want to run after the rabbit or squirrel or to run out to retrieve the duck, or would the dog choose not to put any effort out to get to the rabbit, squirrel or duck.
Check out the following web site about various animal speeds - it's quite interesting: http://www.infoplease.com/ipa/A0004737.html
The site says that over 1/4 mile, a cape hunting dog can run 45 mph, a coyote 43 mph, a gray fox 42 mph, a greyhound 39.35 mph, a whippet 35.50 lmph, and a domestic cat 30 mph. My guess is that a black labrador retriever would run slowing than a whippet, but faster than a domestic cat. That is, somewhere between 30 mph and 35.50 mph
Hope this helps.