Yes, there is such a thing as over practicing. I've done it and was injured as a result. I practice much smarter now. How you may ask? A simple question to be sure but the answer is not.
I once practiced for 16 hours stopping only to eat and use the bathroom. I discovered I could get calluses on my fingertips but I don't consider that an injury or over practicing.
I can now "practice" my way pretty much indefinitely.
My all time record has been 18 hours and without strain or injury. Yeah, read that last part again.
These are some of my ideas for a smarter practice session:
1. I have a plan for each practice session.
2. Long practice sessions require the most caution.
3. For continuous practice (without breaks) I practice one hand at a time. This means that one of my hands is on break. I very often do hands separate practice when learning a piece and switch hands often.
4. Most importantly NEVER IGNORE ANY PAIN and keep practicing. By pain I mean tension, strain, anything at all. You must listen to your body.
5. I work on many different things. Different bars, pieces, scales etc. all according to plan.
6. I take notes.
7. Work smarter not longer. Analyze everything.
8. Breaks are very necessary if your practicing very long or hard. These can be as short as five minutes. Play 25 minutes take 5. Or play 50 minutes and take 10. Get up leave the piano and take in the mail or have something to drink. Phone your significant other. Whatever. O.K. back to the piano for another 50 minutes then another 10 minute break.
9. Does all your practicing really need to be one long grueling session? Marathoners run 10 miles in the morning and then 10 more miles in the evening. These days I rarely practice over 4 hours at the piano and I also don't practice every single day. My hands are all the better for it. When I sit down to practice it will typically be around one to three hours. Then I watch a DVD and/or go for a jog. For you it could be a bike ride, a swim or a trip to the gym or supermarket. Shopping anybody? Then I usually play again later that day.
10. I had to learn to lighten up and be flexible as this is not my nature. I must practice three times a week on Monday, Wednesday and Friday for four straight hours from seven to eleven. This was along the lines of how I used to think. The times, days and hours were "carved in stone". I was (and still am a mental case). A much, much healthier way for me is to look at the results. I play faster, more musically, am learning pieces in a shorter time and look how my repertoire is growing. I'm learning more difficult pieces than a year ago. So, the times, days and hours way of thinking was just my OCD.
11. I may practice at the piano for one hour and then take a one hour break practicing away from the piano. This includes listening to new pieces, reading posts and/or books and analyzing new pieces. I have spent over 4 hours a day just analyzing a piece.
I don't think in terms of an ideal time span to practice. I think instead of goals I am looking to achieve short term, medium and long.
Well, that's me. Use what makes sense for you and ignore the rest, Joe.