When you start becoming serious with your musical education, especially as a pianist, a lot of concern is going into “how many hours do I have to practice”? In the beginning of your college education we often assume, that the more hours we spend at the piano, the better we will become. Gradually most students become more reflected over time, and start to realize that our body and mind has a limit to, how long stretches we can work, and that the quality of our time spend at the piano matters. A lot.
So how many hours? Arthur Rubinstein famously said, that in general you should not to practice more than three hours a day. More and more evidence are arising, that tedious repetitive practice should be balanced by more innovative strategies, like “interleaved practice” (See: http://journal.frontiersin.org/article/10.3389/fpsyg.2016.01251/full) and that the degree of reflection and strategic focus while you practice makes all the difference. No doubt about, that we need repetition to establish the patterns needed to perform the musical works, but in general we probably have to be much more aware of, when to break off from repeating, and when to stop for a pause. This is one of the main results my project FormingPerforming.
Another thing is restitution. If you are training for a marathon, you now that you have to interchange between high-intensity and low-intensity training sessions. And you have to have to take a day off once in a while, in accordance to your overall training plan. This is because of restitution: Our muscles need 24 hours to recover from high-intensity work (and actually our tendons need 36 hours). So if we work to hard to early after a previous training session, we risk to erode our muscle tissue, instead of building it up. This is something that for instance brass players are much more aware of that most pianist.
So the advice must be: Spend time at the piano as long as
1) You are aware and focused on the goal of your practice right now
2) Don’t have the feeling that you are straining your muscles or tendons
And take a break, if 1) or 2) is not the case.