When your fingers stop working. You have been playing things all okay throughout the practice, then you start getting a bit scruffy – basically your fingers getting fatigued.
When you have been practicing for such a long period of time, you begin to doubt what a note is and you start to over think. Then you forget the chord progression to ‘Wonderwall’. Basically you are getting tired.
I think when you have been sitting or standing for long periods of time in the same spot, that can be a sign of “you need to get up and move about”. I know I get the fidgets and lose concentration.
If you are suspecting you are over-practicing, then It’s good to get up, stretch your legs, get some fresh air, get lots of sleep, eat some oats, then back into the practice.