How long does it take to learn piano well?

It depends on your natural aptitude for music and the piano, and how much effort and time you are willing to put into learning.

Your example is odd, since the piano is, unlike many other instruments, a master of chords and polyphonic music. Someone with a good ear could probably play a whistled tune on a piano almost immediately. But that is not being able to play the piano well.

In music education (esp in the uk), the standard for competance is achieving grade 8. Children who practice hard can expect to progress through one grade a year, so about 8 years is a reasonable time for learning to play the piano well.

Proper, structured piano practice helps you stride forwards. Learn here https://tr.im/piano4all

I say ‘proper’ because a lot of people have an idea that practice IS just playing songs over and over. Practice needs to involve unchartered territory.

The golden rule is practice what you can't play, not what you can play