I reckon you're done when...
You're sick to death of your manuscript.
You no longer have a niggling feeling that something's not working.
Feedback from friends and workshop groups concentrates on teeny points.
You've done at least one major re-write which has involved restructuring.
Everybody you know has asked when you are sending it out. And that was over a year ago.
You're sending it out too early when...
You know there's something wrong with the text but don't know what and send it out hoping that no one else might notice.
You know there's something wrong with the text but don't know what and send it out hoping that an editor will see past that. Or better still, do the editing for you.
You've sent it out to your workshop group for feedback and haven't had it back yet, but send it out anyway.
This is your first draft.
This is your second draft.
How do I know? I don't. I stop editing when I stop feeling guilty. I feel there's nothing more that I can do and the text has to go out into the world ready or not.
I think you have to keep in mind the reality of the situation: there is no such thing as a perfect text. Person X may love it, person Y will hate it. You may think it's perfect now; in a year you may feel quite differently. In other words, if you reckon it's done, don't procrastinate by tinkering. Send it out and see what happens.