Yes, with regards to music there is indeed such a thing as over practice. Basically practicing to the point where you are beginning to experience physical and mental fatigue. You start making mistakes where you normally do not and you start to stagnate with regards to learning as opposed to making forward progress. as a rule for every hour I practice I give myself a 15-30 minute break where I put the instrument down and go and do something else.
Then there are those times where I seem to get so enthusiastic and I am playing well and having so much fun that I cannot put the instrument down. I keep telling myself, "that's enough, when i finish playing this piece I'm packing it up for the night". I'll put the instrument down only to find myself playing the same piece or another piece over again 5 minutes later with just as much enthusiasm as before.. Sometimes you just can't stop playing.