-When you stop playing 
Reasonable way of knowing when you have played too much,I find
Or
-when you try and climb into a tree or bush.
-when you jump into your local gym,shouting PIKACHU,I CHOOSE YOU!
-when you crawl around in tall grass searching for a Pidgey
-when you prod around in the air for a while,attemping to open the start menu so you can save your game,switch yourself off to save energy for the day ahead of you
-when you try to get your pet hamster to smash a rock into little bits or move it around

Reasonable way of knowing when you have played too much,I find
Or
-when you try and climb into a tree or bush.
-when you jump into your local gym,shouting PIKACHU,I CHOOSE YOU!
-when you crawl around in tall grass searching for a Pidgey
-when you prod around in the air for a while,attemping to open the start menu so you can save your game,switch yourself off to save energy for the day ahead of you
-when you try to get your pet hamster to smash a rock into little bits or move it around