My teacher ordered me, one week ago, to play just four times 15 minutes a day for now. I have followed that schedule strictly for a week, with the help of a timer.
The point was to make my practicing even more effective than before. I have a lot to do and if I know I just have 15 minutes, I must choose carefully and not waste one minute on silly things.
It has been an interesting experiment so far, and I look forward to extend the time a bit later on. I think I have made good progress this week and I work with better focus and discipline.
Before that I played between 0 and 2.5 hours a day, where a typical session was about 45 minutes before I got tired and had to rest.