When is it time to give up on a ms.?

An agent told me recently that when she looks over her list, the mss. that ended up selling the best are the ones that took longest to place. What's the most number of times you or your agent have sent something out before it sold or you gave up on it?I love those stories of mss. that got rejected over and over, and then went on to become famous books -- here's a list of 14 books that became famous that were rejected over and over by publishers. Some I'd heard of, some I hadn't.....but I find it encouraging, especially the comments that must have seemed idiotic to the authors, like:
"Does anyone drown? [to the author of KON TIKI] Then it can't be very interesting."

But sometimes when a ms. doesn't sell, maybe it SHOULDN'T. Dear Genius has lots of stories about mss. that didn't quite work -- by people who then went on to write other things that worked brilliantly. For example:
"....I don't mean to sound cross, but I felt so hopeful when you left my place last weekend, and was a little depressed by the ms. when I read it (several times) this week...I wish I could be more constructive but until you do more on this it is pretty hard for me to be. I will say this: I think your first "chapter" can't be called "The Tiger," and you can't just say in two lines that this Frances was in bed and she couldn't sleep...."

Yes, it was the first Frances book -- published many rewrites later as Bedtime for Frances.

There is really no way for an author to know how it will turn out or which (worth working on or not) is true of a particular ms....what I have concluded about my own work is that it just depends on how interested I am in the story and TIME usually clarifies that. If, years after I first wrote something, I reread it, see the flaws, and want to fix them, I do. If I read it over, and think: "This is really GOOD! Someone ought to take it!" I keep sending it out, but with a different pitch. And if I read it over and wince, I file it or throw it away, thankful that it was never published.

And on an unrelated note: if anyone read the blog earlier today and saw the post about the "1 weird old tip" -- I was going to develop that into a real post by adding something about curiosity and fiction, but forgot that I had set it up so Blogger would publish it automatically. Sorry about that.

I think that post is a good example of something that wasn't worth working on further!