How long does it take before my 5month old pit/lab puppy loses his puppy teeth?

By the age of 6-7 months your puppy should have his full set of adult teeth.
Sometimes they can retain baby teeth (usually canine teeth) and can be safely pulled while they are under anesthesia for their spay or neuter.

Be prepared for lots of extra chewing and drooling during this time. The pups mouth will be sore and he will want to try and relieve the pain by chewing.
Make sure to provide plenty of chew toys to help loosen the teeth so the pup doesn't take it out on your favourite shoes!

Another member here (catsmine) suggests chicken bullion flavoured ice cubes for the pups to chew and lick to help soothe the gums.