I usually do a complete change once a week. I have two cats and that's about the time it takes. Like Grivusangel, I also scoop the boxes and add litter.
To clean my boxes, I use baking soda and vinegar. It's non-toxic to my cats and rinses clean. And it's about as cheap a disinfectant as you can buy.
I also use baking soda in the boxes to help cut down on the odor. I just sprinkle it on top of the litter and it really helps to subdue the odor, especially if you have a particularly "fragrant" cat.