How often to bring in new genes?

I've been raising chickens for a few years, and in recent years have been just keeping those raised from chicks by their moms. So all my chickens are closely related now. How many years should I let pass before I purchase new chicks to introduce new genes? I have mostly bantams and in the last batch, a couple of the hens are remarkably small. They are adorable as pet chickens, but will not be very useful to lay or hatch eggs. So I may order new chicks in the Spring just to get some new normal-sized genetics in the mix....

There's more to write on that subject than I can get into, other than to say to google animal husbandry and sustainable breeding, you'll probably find more than you ever wanted to know. If your chickens are coming small with bantams on the yard, stop letting the banty's breed. Ive been told by old time chicken and hounds men to introduce new blood into your breeding every fourth generation, (new blood meaning an unrelated individual).

Thank you Tracy!

Research Clan Mating or Spiral mating. This will keep you from inbreeding too closely. Harvey Ussery and others have a lot of info on this.
If you are in the position of having too inbred of a flock, then get yourself a line that is unrelated (provided you are not breeding for Standard of Perfection in a heritage or show breed)
Good Luck!

Thanks! Mostly I'm just trying to have a useful breed of chicken who can forage and raise their own babies. I acquired the bantams specifically to have a source of small feathers, but since they were the most prolific breeders, they have taken over the flock. That rooster gets retired to freezer camp this Winter, and so will several of his little harem.

Just get rid of all the existing roosters except for one. And then once the new rosters have matured and settle in get rid of that last one.