How long before I should start planning holding back future breeders?

I was planning on holding back 4-6 girls and then next month hold back a male or two.


I currently have 1.3 breeders and my girls are now 6-7 months old, they've been dropping litters every 5-6 weeks. Litter sizes are between 8-12 and I've had no problems at all. I also leave the male in with the babies and have had no problems. I'm even thinking of adding a 2nd group of 1.3.

P.S.
I also got my 1st bite last night, I went to feed them and drop in some cheerios as a treat and one of the girl mistook my finger for a cheerio! Ouch!