My Male Sneakers, retained his Baby K9's for almost a year, WHILE his permanents grew in ahead of them. The whole time there were rock solid, and I had to have them extracted along with a along with a couple other deciduous, during a regular Dental Cleaning. He just didn't want to give them up.
My Female Becca, on the other hand, dropped all of her baby teeth on her own. She's 2yo now. lol
PS: The delay in removing Sneakers K9 teeth, was recommended and under the supervision of our Vet. I was a lot more nervous about therm being there, than the Vet was.