The electrodes are intended to be replaced regularly to prevent bacterial or other buildup, or degeneration of materials. With daily use and regular cleaning of electrode pads, they will typically last for 4-6 weeks. Your electrodes should be replaced if there is significant discoloration, bad odors, or any noticeable separation of materials.
CAUTION: Inspect electrodes prior to use. Damaged or dirty electrodes will affect stimulation efficacy, and hence should be replaced regularly.