120 adults?

It's simple.

Let's say an adult rat is 16 weeks old. That means you need to pull aside 120 rats every week for sixteen weeks. Mind you now, during this initial 16 weeks NONE of the rats can be fed to your snakes.

The first week you'll have 120 weaned rats. The second week you'll have 120 4 week old rats and 120 weaned rats. The third week you'll have 120 5 week rats, 120 4 week rats and 120 weaned.

See how that grows?

By the time you hit your 16th week you've got a BUNCH of rats growing but then, on week 17 you will have 120 adult rats every week for as long as you keep the line going. Of course weather, malfunctioning equipment, newb mistakes on your part, can all set you back weeks at a time.

Good luck though, I like it myself.