Answer: As many times as it takes.
For me, I normally do about five 'Final Drafts'. It's crazy. Each time, I save the next draft as 'Draft_FINAL' and it never is. There's Draft_FINAL One'; 'Draft_FINAL Two' and so on. My personal favourite? 'Draft FINAL_FINAL'.
Each time I review a draft, I find more errors, or more room for improvement. Each re-draft takes less time than the one before, but each re-draft is harder, more freaky, because each time I re-read it I realist that there's still so much wrong.
So I suppose for me, a piece is finished is when I can't bear to read it again. If I work on this story anymore I will go insane.
And then I take a deep breath, package my finely crafted work up and sit it on it for four to six weeks.
I dread the next part, the sending it out into the wide wide world. Because as soon as that happens, I start getting rejections.
Rejections are almost inevitable. But, oh, how they hurt. So instead, I keep working on a story, trying to make it perfect, delaying the inevitable pain.