How fast will my corn snake grow. Is it worth ordering pinkies in bulk?

I have a young corn snake, about 12 inches long and weighing about 8 grams. He is currently eating one f/t pinkie mouse a week. I'd like to save money and buy online. Here's the thing though. The smallest quantity I could find was 25 mice. I'm worried that if I buy them, my snake will be too large for them before I finish them. So my question is, how fast will he grow and need to be moved up to a new size? I don't want to end up with a freezer full of useless mice. Any suggestions?