When carefully handled, electrodes and caps can last for years. Exact lifetime is really subject to care.
Before concluding the electrodes are malfunctioning, please check out this article - I'm having trouble mounting some electrodes - they're showing NC (not connected) or have very high impedance.
If you encounter an issue with an electrode on a Smarting Base cap, this can be replaced remotely, with some guidance from us. With the PRO line caps you should contact us for further information support@mbraintrain.com