How long will these last me?

On average... you can expect a single rat to eat 1lb per month. But it depends on alot of factors. I'd probably guesstimate you will go through 3.5-4lbs a month.

I'm not sure on the bedding.

The best idea is to run through a month or two and see how much you used.

I know I'm not answering your question but, I'm curious. How do you keep the bedding in a critter nation? The pans are shallow?

I put the litter in litter pans, and they do not kick it out. Not my doing, but it appears that my girls like the rocks I put in with the litter and are peeing in the litter boxes as well as (mostly) pooping there.

If yours aren't litter box trained, and you are using litter throughout the trays, there are Ferret Nation sides which clip onto the trays of a Critter Nation to give them a lip of a couple of inches.

I got mine herehttp://www.petsupplies.com/item/ferret-nation-scatter-guard/650415/, but haven't needed them as yet.