Celeste makes an excellent point. If your mind is focused on the composer's intent, interpretation, and attendant technical details, and/or even forming mental imagery that helps you to project the piece, that's true concentration during practice.
On the other hand, if what are entering your mind are completely extraneous thoughts (doing an errand later, unthawing the vegetables, checking email, tonight's date, etc.), that means that it's time to stop. No further value will come of the practice session. Worse yet, with your mind bifurcated between practice and unrelated thoughts, you'll be committing errors that will undo your progress. Tomorrow is another day.