I love hate am working on it.

Revision is a funny thing. I don't know how I feel about it in the end -- it can take so many different forms. Of course I do it, and I am not against it, but how much should one revise? Also, how should we approach it?
Somewhat ironically, my writing is similar to my revision, and there is a very fine line that houses the best balance of guidance and ability to revise my work. What I mean is, if I'm too guided in my writing, usually it's not as good as I'd like. But if I'm just given a prompt and maybe one or two other ideas that are left vague-ish, then I can turn out something a lot better. Go too far and have no limitations, however, and I get annoyed at taking so long to get started, and inevitably am not up to snuff. And it's the same with revision: if I have too clear an idea at first, there's no room to expand on it; I can add a few lines here or there and maybe a scene most likely just there for character development, but nothing that truly adds to or does that "double-duty" for the play at all. If I started out with a more vague idea, however, I have much more room to play. I can decide where something else should go or that some part needs more build-up, etc., etc.
But at that point, when do I say stop? How do I discern when I am adding good stuff to the play and when I'm going overboard or straying away from the point? I think knowing your characters sets this guideline. If you start out with an idea and then can put very concrete characters in it, the writing will be relatively easy. For the most part, I think this is what Peter Schaffer was able to do in Amadeus. And while he did run in to a problem with the ending, he was trying to find the best theatrical one -- he knew the characters very well and was trying to fit them into something he could see in the play as he went back and examined different themes that had surfaced that would most fit the characters in a climax.
But still, in terms of the real world, where do we cut ourselves off? I get kind of annoyed with people who say a work is never truly finished, or the only reason it's not changing any more is that they got tired of changing it. For the most part, I would say being published is the cut-off point: you're good enough to get through an editor and have people buy/produce/whatever your writing -- that means it's good.
In a case like Schaffer's with Amadeus, though, I don't know... I mean, it was good, yes. But he had a clear idea of where he wanted to go, just not exactly how to get there. And people were willing to workshop it, so...
Who knows. I've never published anything, so I can't say if I'd want to work on something that long, OR come back after ten or twenty years and work on it again. What I'm trying to say is that "flaws" you see later in a play are fine, but if you have a purpose, say, a "director's (well, author's) cut" -- a new ending, a theme to bring more light to, whatever -- then I think those twenty years are acceptable.