When I get tired of it?

 So hard to know when something is finished. I read recently it's when whatever you do next won't improve the painting. Makes sense, but how do I know that? I'm not usually so confident in my work that I feel it couldn't be made better. 

Here are 3 intervals at the end of a painting session. So many times at this point I do dozens of small, probably unnecessary things that sometimes even weaken the work. I stopped because I was tired of the painting, not because it couldn't be better. Perhaps I need to go with when whatever I do next won't significantly improve the painting. . 

How do you know when to stop?