I have two aphorisms that cover this, and like much that can be found within my mentatarium, they are mutually contradictory.
No one cares about your work more than you do.
The writer is the worst judge of their own work.
I am cheerfully capable of integrating two mutually contradictory ideas into my head on a running basis. (That's pretty much the definition of being human.) However, I think knowing when it's good is really a question of knowing when to stop mucking with it.
My theory on this is pretty basic. My first draft voice is my strongest voice. Rewrites are for actual boogers, minor line edits, fix-it notes, and to close plot holes. Polishing is the death of voice, for me. (There are rare exceptions to this in my process, for stories with serious breakage.) I improve as a writer by writing new stories, not by niggling at existing stories. So how do I know when it's good enough?
First, by not expecting perfection.
Second, by not plotzing with the manuscript any more than necessary as a general rule.
Third, by writing another story and seeing if I got better.
It's a rare writer who improves their voice by polishing. Write more, and get better that way. You acquire new inventory and have more fun. Besides which, self-editing can be the most justifiable form of cat-waxing.