It depends on what type of bedding you are using. The wood shavings or that fluffy cotton stuff will start to stink in a matter of days.

The aspen pellets (which I can only find at petco) lasts forever and you have absolutely no rodent smell at all. All you smell is the bedding and it smells really good. That stuff only needs to be changed every 2 to 3 weeks depending on how many rats are in that one cage.

I recently switched to the wood pellet bedding for livestock. You get it at big farm & feed supply stores. It comes in huge bags and only costs about $12 to $15 depending on where you go.

It is a mixed wood pellet so it doesnt smell as good as the 100% aspen pellets. (has almost no odor) it tends to crumble & swell a little faster than the aspen pellets too but it lasts almost as long and is alot more cost affective.

The bad thing about the wood pellets is its dusty. Gets rid of odor and is extremely absorbent but for nesting females you would want to put a couple handfulls of shavings in for them to make the nest.