When it's been reviewed to death by your writing group, and there isn't anything you can do to make it better.

There are always more things to do on novels and stories and poems.  It will never be perfect.  If you feel no more enthusiasm for your work, it's time to get some other people's opinions (like editors or agents).  I say go for it!  Research your market and send out your manuscript.  If you never send it out, it'll never be ready for market.