Well I have a smart answer to that, which goes "When it's sold"! But then again, for most painters, I suppose there's some truth in it, and, in my case, quite a lot. I've often returned to a painting months after I initially deemed it finished and gone on to make significant changes. But usually they are only fairly minor tweaks. In the case of the one below, I've fallen somewhere in the middle following some constructive critique from painting frineds.
So hopefully, I find a buyer quickly, because I think I've tweaked this one enough!