it depends upon the breed how fast they will grow.
we have a belted galloway that we got with calf; she had that calf and he's growing SO slowly. we're ready to butcher him this fall, so it's been (my brain not working so well) almost 2 years at that point. we also had a calf that was belted galloway bred back to angus and that calf is already as big as the straight up galloway. angus grow pretty quickly, which is why they're the breed of choice for so many folks.
dh works pretty hard to maintain our pasture, fertilize with nitrogen, etc. and it dies in the winter. we don't have anywhere to grow hay so we have to purchase it... therefore he calculated that it was more cost effective to let the bull get big before butchering it and timing it to butcher before the grass dies. you can always butcher early, you know.. it just depends upon what you have to feed it and the cost of feed vs. grazing, etc.
you also want to time it well.. spend some time researching breeds and thinking about cost. we know folks who will buy holsteins because they practically GIVE the bulls away from the dairy farms, but they don't dress out well. i think you can do those in two year's growth.
this is a good article on raising those from backwoods home.
take a look at the storey's guides. that should help you decide, i think...
here's a chart on frame size vs. weight at maturity (but not length of time to maturity)
Edited by hildare - 8/12/11 at 12:19pm