How to know when your painting is finished.

Best pearl of wisdom I ever heard was 'if it doesn't look good yet, it's because you haven't finished.'

Just keep going and going and going until it looks how you want. 

What do I do?

A different perspective is always useful. Sometimes I'll view the painting through a mirrors reflection to try and see it from someone else's eyes. I also put the painting to one side for a few days so when I see it again, it's through fresher eyes.

There are plenty of paintings I've finished too soon and plenty that I've overworked. If you're using a reference, it's much easier to determine if it's 'close enough' but painting from imagination is a bit more challenging. Practice will give you the experience you need. 

Take a look at this feature from Jacksons Art for more advice.