I got this out of a vet book:

Pups begin to aquire permanent teeth about 4-5 months of age. Usually last for about 2 months.

Good luck in finding the teeth. In all the dogs I every owned
I was only able to find a couple of baby teeth from two of them.
Some people on Pet Talk have been quite lucky finding
their pups baby teeth.
Mine must have swallowed theirs.