Its a broad a question to ask how many litters / how long is a girl kept for as every breeder is different, for example I neutered 2 girls after 1 litter each and they both went to live with people who had kittens from me already, one didn't like being a mum or around so many cats, the other had a really traumatic birth and I didn't want to put her through it again. Then I have a girl who had 3 litters, loves everyone and pings back into shape! Although I do know of some breeders who have cats that are a lot older than that and in great shape.
But I don't like my cats outside so I try and make the best home for them all inside with me and the ones I have I bred myself so all were brought up together, although they are like brothers/sisters have spats generally they all get on and the house is theirs! Lots of places for them to get away from each other if needs be.
I always said that I wouldn't rehome any of my girls, but when it came down to it I either keep them all and stop breeding, or let them go to a great pet home like I would do a kitten, and they have all gone to great homes, of course it is heart breaking you watch them come into the world bring them up watch their own kittens come into the world
I still think about them everyday and chat to who they have gone to live with and all are really happy, still doesn't really stop the guilt!