Hey Klaus, your days will vary dramatically as you are learning so it is important to find what exactly you can do consistenly, day to day, no matter how much you play.

Can you play an effortless, resonant low C every day? If so, a D and so on when. At what stage does it all fall apart? This is all a part of your awareness development.

The best way to go I think is small sessions many times a day with lots of rest. However, I believe there are benefits to be gained playing until you can’t play any more; just do this every so often, not every day otherwise you will constantly be playing on fatigued chops.

Best Wishes,
Greg