Bach's well tempered clavier

[tommy]

How many months/years playing piano before it is advisable to start using Bach's well tempered clavier?

[Hmoll]

First of all, it won't be months. It will be years.
Secondly, it is not counted in time at all, but in preparation.
A better questions would be "What Bach pieces prepare one for learning preludes and fugues from the WTC, and what are the technical and musical demands of these pieces?"
The pieces that lead up to WTC are pieces from Anna Magdalena Notebook, Bach's little preludes and fugues, two part inventions, three part inventions. Also, there is a wide range of difficulty and technical requirements within the WTC, so some of the less challenging ones in these sets lead to the more challenging ones.

[bernhard]

Hmoll said it all. So most of what follows is a repeat.

It is not a matter of time.

There are only two kinds of pieces: easy and impossible. The difference between them is correct practice.

Try the WTC. Is it impossible? Then we have the good fortune that Bach paved the way for us to get there.

In his own teaching he created the pedagogical materials that would lead to the WTC. In progressive order of difficulty:

1.      The little notebook of Anna Magdalena Bach
2.      The several collections of little preludes and fughetas written for Wilhelm Friedemann Bach (his son)
3.      The 2 voice inventions
4.      The 3 voice inventions
5.      The French and English suites
6.      The partitas.

Usually if you can play the two and three voice inventions, you are ready for the WTC (in fact some of the three voice inventions are as difficult as some fugues in the WTC)

Also bear in mind that some people seem to have a natural facility for  contrapuntual music, while for others it is an uphill struggle.

Best wishes,
Bernhard.