There are two factors that you'll notice that should ring some alarm bells that you need to replace your tea kettle with a new one, these are:
Your water starts to taste strange
A build-up of minerals in your kettle can start to alter the taste of your boiled water. If you start to notice a metallic taste, it’s probably time to get a new kettle. Regular cleaning/descaling can reduce the build-up of these minerals and increase the lifespan of your appliance. No one likes crunchy bits at the bottom of their teacup.
Your kettle is taking longer to boil
The heating element in your kettle can lose efficiency over time. This is primarily due to limescale build-up, particularly in hard water areas. If you notice that it’s taking longer to make your morning brew, it’s probably time to replace the appliance. Regular descaling can help to prolong the shelf life of your kettle.