It's an interesting question to be sure. When do you drop the quill and ink (err... I mean mouse and keyboard) and declare finito!?
When you've finished your "final" version?
When you feel "satisfied" with it?
When your cat stops meowing and sleeps during the music?
When your peers at Composers' Forum shower you with praise?
Isn't there always something else you can add or revise?
Personally, I agree with Leonardo:
No work of art is ever finished; only abandoned - Da Vinci