Life of a writer. Episode 37 (or something).
I can always tell when it's time to stop writing. It's when I start procrastinating, searching the web for images (the excuse being "I need to do research") or getting another cup of coffee or tea (the excuse being "I need a snack") or starting to think of other things besides my story (the excuse being "I need to do chores"). At this point, if I force myself to continue, everything I write will be shit and I will have to rewrite it the next day anyway so I stop and breathe in and breathe out and call it a day. For writing, that is. Which is usually 4 hours and about 2K words.
Then I read. For 2 hours or at least 100 pages.