The answer to 'when to replace water softener' is that you should generally replace your water softener every 10-15 years, or sooner if you notice it's not working as effectively. Water softeners have a limited lifespan, and over time the internal components can wear out, reducing the softener's efficiency.
Some signs it may be time to replace your water softener include increased water hardness, reduced water pressure, and a decrease in the softener's salt consumption. If you're unsure, it's best to have a professional plumber inspect your system and provide a recommendation.