As much as it takes until I can play whatever consistently and musically from rote recall upon demand without other than an occasional minor mistake as occurs in anything one does. Where a song, that includes regular review, revision/reinforcement as required of my current set list/favourites to a point I can play any of them impromptu consistently to that same standard. Although a left brain dominant, I assess my ability to perform musical competencies qualitatively, not quantitatively, as it must be IME&V.