When the Rods are worn and ready to change the fault light will appear to say "CHECK IONISER RODS".
The fault will only show during the time when the Ioniser is actually working, this will vary depending on the size of your pool but will typically be during the first few hours of the T1 Timer period.
The Copper/Silver Rods are sacrificial and will have to be replaced from time to time (typically every 3 years) depending on the size of your swimming pool.