Concept is your compass. It is not true that you never know when a painting is finished, if you have a concept, you will always know. The direction that your painting lead you in may be different from what you had planned but because you had a concept, you have had the tools to evaluate the conversation the whole way along.  You know when your painting is finished because you took the time to do the hard work and to figure out the WHY? of the work.

The understanding of your concept is part of being aware of your aesthetic as a painter and understanding what you are trying to communicate, whether it is an obvious narrative or a subtle mood.

There are a couple of articles which will hopefully make this intuitive area a conscious tool for you.

These articles on concept may be of value to you