All solids must be removed regularly each day. The litter may need to be topped up weekly to compensate for this removal. The urine is absorbed into the crystals and they will become clear when they have reached their absorbency capability. The urine will begin to pool to the bottom of the tray once the crystals have been used to capacity. We also recommend the entire tray be changed, washed and filled with fresh litter at least once every four weeks. The crystals should last up to four weeks with one healthy cat if the litter is used correctly.