Why do I have to press the power button twice before the laptop turns on?

That's the neat part: you don't.

The power button does not have to be pressed several times - it only has to be pressed a little longer on the first attempt than one might intuitively assume. This circumstance has a technical background in the circuitry and signal propagation time and, as a side effect, is also intended to prevent accidental cold boots.

How long do I have to press?

The exact value of how long the key must be pressed depends on the respective laptop model and its key component configuration. Rule of thumb: a keystroke duration of about half a second should generally be sufficient.

I always press very hard... (please don't)

It is not neccessary to press the button particularly hard. It is a digital switch - it only knows "on" or "off". However, if you press firmly, you naturally tend to hold it down longer and you might then come to the assumption that strong pressure is necessary. But this is indeed not the case. Fact is: only the length of the keystroke is decisive. So you should ideally press the key gently, as long as you're able to hold it for a moment before releasing it again. Then the laptop will switch on reliably.