One bag will last the average cat about three weeks under ordinary circumstances. One bag lasts one cat one month*. This claim is made by almost every crystal litter out there. It works like this: First of all, the litterbox should be filled to a depth of two inches (8 cm) for the crystals to function optimally. So, if you have a small litter box and a small cat or kitten, one bag will last about one month. For a great big cat with a great big bladder, it will be less. Also, individual habits vary widely in regard to how often the litter is changed. Damp environments, such as a bathroom where the shower is used often, shorten the useful life of the crystals. So the 1 bag 1 cat 1 month claim is affected by many variables and is true only in ideal circumstances.
*The more accurate way to state the case would be to say that if the litter box is filled to a depth of two inches (using more than a 4Lb. bag in most cases) and is stirred about regularly, it will last the average cat about a month, and will give you far superior results in terms of an odor free home environment for your family, guests and pets.