Most artists that I speak with agree that knowing when to stop working on a creation is often challenging.  There are instances, to be sure, that you just 'know' it is done, or when circumstances put an end to the project for you;  such as running out of daylight, a collectors time-line for delivery, or the materials drying so as to make further work in that manner impossible. 

For all of the other times it is entirely subjective, and for me it seems to be mood dependent.  I try to create original art in a pure sense, and for me that means no agenda, no reference to refer to, and often times not even a theme or subject defined in advance.  What my art becomes is figured out within itself, the physical art and the act of art meet in the medium.  In this sense the art is always in a state of becoming, like the life of the artist himself. 

Without a reference to compare the work to it potentially becomes a never-ending project.  Ultimately it takes practice to learn what is too little of a good thing, or too much of it.  My art is meant to be expressive, so I tend to end up closer to the 'too much' spectrum. 

I find that when things start looking 'out of place' when the painting is more mature in progress, then it may be a good idea to at least put it aside.  A fresh set of eyes on it the next day, or longer, may be all that is needed to decide on 'doneness.'