The time needed to make the Levain depends a lot on the yeasts themselves and other factors like the Flour used and the temperature at which you let it ferment. For this reason, there is no fixed time.
With the method I describe on Biancolievito, you can obtain a good levain in about 8 - 10 days.
To facilitate your starter's production, I recommend you be constant in refreshments timing, in the Flour used, and in the leavening temperature!