As long as it's enjoyable is my answer. Quantity need not dictate advancement. When I first learned violin at 15, I completed six RCM grades in a year on 3 hours a week of ensemble play and about 1 hour of solo play. The ensemble was a total delight, and the excitement at hearing the harmony of the group provided motivation for solo work.
Strangely, I find time thinking about the music in my head outside practice was what really helped me progress. I have a strong auditory memory and can easily hear simultaneous parts as though they are played solo, so deconstructing and reconstructing the group play was what helped me better understand the role that my part played in the ensemble. It gave me a heightened appreciation for the dynamic range required to optimize the listening experience of the audience. I have to wonder if others here/others' children use a similar non-practice approach to working on a piece.
On the upper end of the scale, if I was caught by the composing bug on piano, I'd often go most of a day playing, sometimes getting so caught up that I'd miss meals (to my mother's chagrin). Those are the days where I feel music in my soul and need to release it, like a breath trapped in my throat.
So somewhere between 30 minutes and all day to get to a teaching level of proficiency (ARTC in RCM) in 3-4 years. What a useless recommendation, isn't it?
