From my experience, fixed session every morning (15 - 20 min) plus a weekly teacher lesson works well enough. Short as it looks, this makes more than two hours a week of the most productive time, with nobody distracting.
With weekly piano lesson from the teacher in addition, this seems is enough to make progress that is satisfactory for me. Started from scratch a year ago, no can play many probably simple but rather nice songs from my textbook.
May take about 200 - 300 years to get to the Rubinstein level this way but hopefully this is not the goal.