A softener removes hardness because the resin beads inside the tank trade the hard minerals for sodium. That exchange only works while the beads are clean and free to do their job. Over time, what arrives from your supply decides how well they keep working, and that matters more to the long run than most owners expect.
What fouls the resin
Iron, sediment, and oily residue can coat the beads and block the exchange. Chlorine is a different problem, because over time it can break down the resin itself. If your supply carries any of these, the softener may still run on schedule while its softening ability slowly drops.
Signs the bed is losing ground
Watch for hardness that returns sooner than it did when the unit was new, or for a regeneration that seems to end before the water is fully treated. A rise in the amount of salt needed for the same water use can also point to a bed that is no longer working as well as it should. Note these changes in writing so you can describe them clearly to a service provider.
Questions to ask before you buy
Ask the provider whether a water test was run on your supply and what it found for iron, sediment, and chlorine. Ask whether they recommend pretreatment for any of these, and what that pretreatment would need in upkeep. Ask how the unit is set up to handle the conditions they found, and whether that setup would still work if your water changed later.
What you can do yourself
Keep a simple log of the regeneration days, the salt you add, and any change in how the water feels or tastes. A steady record makes it much easier to tell a normal pattern from a bed that is wearing out. If your supply is from a private well, retest it on a regular schedule, because the conditions can shift over time.