1. When you've done everything you can think of to the novel.
2. When you honestly have no niggling feeling in your stomach that there's something wrong somewhere.
3. When your writing critique friends are telling you to get on with it, it's good to go.
4. When deep down you know you're putting off the moment because you're worrying about rejection.
5. You're sick to death of it.
This is of course assuming:
1. You've finished it.
2. You've put it away, then come back to re-write with fresh eyes.
3. You HAVE come back to re-write - at least once, if not twice, thrice....
4. You've done some research on suitable agents and have a target list to send out to.
5. You've asked for feedback from writer friends.
6. You've had some professional feedback (if you can afford it, you really should get professional feedback).
You don't want to send it out too early, before you've re-written it, but on the other hand, don't wait until the right moment, or when it's perfect. The right moment will never come, and no novel is ever perfect. If you've done the work, let it go...