I used to make kefir, just sort of lost interest, think I'll order a new culture though... It's great for lactose intolerance, something in the kefir eats the lactose. Watch sealing the containter for too long, I came home to a cracked glass bottle when I would naturally carb it. You'll get to know you're culture.
It won't grow in size but kefir grains can be used to ferment other things to add a twist to a beer or cider.