For most versions of Arm & Hammer cat litter, you should scoop it out at least once per day. If you have multiple cats using the same box, you may need to scoop out waste a few times per day. This is true for sand, crystals and Arm & Hammer naturals litter. Add more litter as needed to compensate for the scooped waste.
The litter in your box should be completely changed about every 3 to 4 weeks. Even litter that clumps can allow some waste to collect at the bottom of the litter pan. Liners are an easy way to swap out litter—just tie up the liner and throw the entire litter contents away. You may not need to change the cat litter as often, depending on what kind of cat litter box you have.