FAST. they will grow several inches in the summer months alone when they are actively feeding (outdoors). Once they reach about 28+ inches the growth rate really slows down
yup, if you live in an area that's too cold, or your pond is too shallow (should be at least 3' for koi anyway) you'll probably want to move them to a tank for wintering. if yuo can get a pump or heater yo keep some of the surface un frozen, you could prolly leave em out all year.
yeah they go dormant . there is quite a bit of debate over the hole wintering koi thing .if you heat a koi pond over the winter then the koi will grow about double or more compared to an unheated pond in a cool area