I just heat water in the microwave.

All you need is vinegar and water.

If you don't like the smell of vinegar: A 5kg bucket of food-grade citric acid powder costs 20€ and will last you a lifetime. I literally bought mine when I was an apprentice over 20 years ago in my first one room apartment. It has moved nine times with me, it is half empty by now so when predictions are correct, I'll have to get another one for retirement.

Fill the kettle, add a teaspoon of citric acid, boil for a second and let it sit. You'll see small bubbles rising where the limestone deposits are being dissolved. That fizzing is carbon dioxide escaping: limescale is mostly calcium carbonate (CaCO₃), and the citric acid attacks it, breaking the carbonate apart. The calcium gets bound up by the citrate into calcium citrate, which washes away, while the carbonate leaves as CO₂ (the bubbles) and water.

Roughly, the reaction looks like this:

3 CaCO₃ + 2 C₆H₈O₇ → Ca₃(C₆H₅O₇)₂ + 3 CO₂↑ + 3 H₂O

Citric acid is a weak acid, so it happily eats through chalky scale but simply doesn't react with stainless steel, glass, or plastic, whatever the dose. The one teaspoon rule isn't about protecting the kettle, it's just about not wasting powder and not having to rinse forever. Use more and it works faster and fizzes harder.

The one exception is bare aluminium: citric acid can dull or lightly etch it, so don't use this on an old aluminium kettle or pot.

Once the bubbling stops, the reaction is done: everything that could dissolve has dissolved. Pour it out, give it a quick rinse or the next coffee will have a sour note, and the kettle's clean, no vinegar smell.

EDIT, because I only now read OPs post properly: I've only used this on electric kettles - I wouldn't even know any other ones, I mean, unless you count the ones you put on a burner at a festival or something like that. Those never saw enough sustained use to build up limestone for me.

You need to choose your kettles carefully and get one that is easy to clean. Or just don't look. The iron and calcium won't kill you.

Vinegar, water, let it sit over night and wash it out the next day.

Or just ignore it, it's not like it boils water noticeable less effective.

We have a bosch glass kettle and they sell this expensive kettle cleaner.

We found out you just boil a cut in half lemon in there and it comes out spotless.

Citric acid.

Drop a teaspoon of citric acid into a full kettle, boil it, empty it, rinse it out, repeat if needed. That blasts away the build up. It is how high end kettles like Fellow stag recommends cleaning their kettle too because you can't scrub its mirror finish hotplate.

I just boil a tablespoon or two of vinegar. Unless you let it get totally out of control any weak acid will work instantly, it's food safe, and I already have vinegar in my pantry. Rinse twice or your next coffee/tea may taste off.

I have an electric kettle at work and a stove top kettle at home, in an area with incredibly hard water. I just rinse them occasionally and some flakes come out. Why care? They still work fine to boil my water. The electric kettle is only 2 years old, but the other one is about 20 now and still is fine.

I just use vinegar

So glad I live in a soft-water area. Weirdly, the local supermarkets still sell limescale removal products. No idea who's buying them.

I only use reverse osmosis filtered water in mine, no deposits!

Can also help to pour out the remaining water instead of letting it evaporate.

Tbh doesn't sound that hard, unless I'm missing something in the description.

What would make the stove-top kettle easier to descale?

Damn, you must have some really hard water. I've never really felt the need to clean out any of my kettles.

What's a "regular" kettle?

so vinegar alone was not cutting it. works for me usually but our water is not that hard.