That depends on what they do. If they are to be kept only as pets, and you keep on top of routine things like foot trimming, worming and vaccinations then they can live to be 17 or 18. Most don't live quite that long, but it isn't unheard of. What shortens their life span is the "work" they do, ie if the female has kids every year the strain of doing that and producing milk can shorten the life span a bit.
Beth