Hi, we keep two breeding cows but we don't eat them, we eat their bull calves.
So we bought the girls in October 2010 at 6 months old; they calved for the first time this May / June 2012, one had a bull, the other a heifer; we'll run the bull calf (castrated) on to just short of 30 months, so end of 2014. We could sell the heifer at weaning to keep numbers down. We bought a weaned bullock last autumn for beef.
The cows are now running with the bull to calve June 2013, at which point we will have two cows with calves at foot, Henry (26months), George (12 months), Annie (12 months) if we don't sell her.
So you can see how the numbers grow. We have Shetlands, so it's not unusual for them to have 10 calves, so we will simply keep the two main cows for as long as they are fit to breed.
Hope this helps.