how often should i change my cats litter box?

The instructions on the litter say to completely change the litter once every 7-10 days. However, I haven't seen the need to do it that often with my cat. i scoop the litter every day, and add some to it when it gets a little low (1-2 times a week). Depending on the brand of litter, and how dirty it gets, I completely change the litter every 2-3 weeks. You'll be able to tell when the loose litter is getting dirty and stops controlling the odor as much, typically that is when you want to completely change it and wipe out and rinse the litter box with warm soapy water.

Depends on what kind of litter you are using. I use the scoopable type and change the whole litter about once a month, but I keep putting in some new litter each time I scoop out the used litter (urine makes the clumping litter turn into balls so you can scoop them out easier). Regular clay litter should be changed once a week or the bacteria begins to proliferate. Other types of litter, just follow what's recommended on the bag/box.