B.F. Skinner and other experts have found that the mind's ability to learn drops
significantly after prolonged intense concentration. Research shows that studying too long (i.e. more than four hours) can deplete chemicals in the brain necessary for learning. Therefore, it is best to take frequent breaks (a 5 minute break about every 20-25 minutes) and practice no more than 4 hours consecutively.
By applying these techniques, you can dramatically improve the quality of your practice. You'll be able to use your time more efficiently and increase the effectiveness of your practice.7 Tips for Effective Musical Practice