This was written by a violinist, but is very applicable to pianists. Deliberate, intentional practice is so, so important to musical improvement! When I was in college, I practiced 8-10 hours a day during some periods, and I paid for it in unpleasant physical symptoms. Practicing too much is an issue, especially for more advanced students. This article is well worth the read.
http://www.bulletproofmusician.com/how-many-hours-a-day-should-you-practice/
http://www.bulletproofmusician.com/how-many-hours-a-day-should-you-practice/