As far as a vet visit - most breeders require you to take the cat/kitten to your vet within the first 2-3 days of you picking up the cat.
As far as how long a wait - that depends on a lot of things. When the litter is due, how old the kittens are and what specifically are you looking for (color, sex, pet, show). The wait can be months or years
We are on the list for a show Ocicat male - cinnamon or cinnamon silver. I'll willing to wait for the exact kitten.