For me, it's just the time dedicated to smoothing everything out, adding the fine details, and having the patience to not throw it in. I heard this expression, I do not know the exact words, but it is something like, 'a painting is never complete, but instead abandoned.'

For the artwork you linked, it would be a matter of defining the forms, refining the lighting, and basically, just sticking to it. I've often agonised over my works, thinking 'how do I do it,' but every time, the answer simply has been to work on it for longer.