Why descaling matters
Limescale builds up in any machine that heats water. Hard water makes it worse. The scale coats the boiler and narrows the water paths inside the machine, so it runs cooler and slower than it should. Left long enough, it kills the machine.
Descaling dissolves that mineral build-up. Do it on a schedule and your shots stay consistent. Skip it and the first thing you'll notice is the coffee getting worse; the second is a repair bill.
How often to descale
Every one to three months is the usual range. With hard water, lean toward monthly. With filtered or softened water you can usually stretch to three months.
Many machines have a descale light. Treat it as a reminder rather than the only signal, because water hardness varies a lot from one town to the next and the machine is guessing.
The step-by-step routine
1. Empty the drip tray and remove any water filter from the tank.
2. Fill the tank with descaling solution mixed to the ratio on the bottle (or follow your machine's manual).
3. Run the solution through the group head in stages, pausing to let it work, until roughly half the tank has passed through.
4. Run the remainder through the steam wand and hot-water outlet if your machine has them.
5. Refill with fresh water and run at least two full tanks through every outlet to rinse thoroughly.
What to use
A dedicated descaling solution is the safe default. It's also what the manufacturers tell you to use, which matters if your machine is under warranty. Citric acid works too and costs less.
Be careful with vinegar. In machines with aluminium boilers it can attack seals, and the smell hangs around in your next few shots. Only use it if the manual says you can.
Slowing scale down in the first place
Descaling treats the symptom. Your water decides how often you need it. Filtered water, a tank filter, or bottled water blended for coffee will slow scale build-up a lot, and many machine warranties quietly assume you use one of them.
One caution: don't use distilled or zero-mineral water in machines with electronic water-level sensors. Some of those sensors need minerals in the water to detect it. Lightly filtered is the sweet spot for most home machines.
If flow stays slow or steam stays weak even after descaling, scale may have hardened deeper in the system. At that point a proper service will do more than another descale cycle.