When is a painting finished?

One of the hardest things for me (besides choosing a subject to paint) is to know when it's finished. I started to correct my mistakes - but after a few little brushstrokes decided that I like it just the way it is.
If I were to do more I feel like I would overwork it - and perhaps ruin it. So here's my final portrait of Mark Landis.
By the way, they posted the portrait on the Art & Craft Facebook page. You can see it here:
https://www.facebook.com/artandcraftfilm?fref=nf
Art & Craft

I watched the best documentary I've seen in a while the other night.
It's called "Art & Craft" and it explores the forgeries of Laurel, Mississippi native Mark Landis.
Landis' crime was (in my opinion) fairly harmless because he didn't profit from any of his forgeries - he was being a philanthropist.
The thing is... the guy is really a great artist.
Anyway, I did this really - really quickly tonight. Just basically throwing the paint on the canvas. I plan to refine it more - I see about a hundred things I need to change. But I wanted to post a before, so I can gauge the "after"!
By the way, I'm planning to send this to him.