I keep mine for 12 weeks...
It is very important that puppies stay with their mothers and litter mates. This way to the pups are fully vaccinated and gives me peace of mind as a breeder. This also allows my puppies to be under the vets eye 4 times.
Thank you feminvstr for posting this educational info on development of a pup. 
If a tiny is in the litter I will hold them as long as needed to make sure of health and development.