I think you have to judge each book separately and trust your gut.
I used to think you had to finish everything.
Then I realised if a book really isn't working, editing bits here and there over years is a waste of time when you could be writing something new.
Then I realised a bit of perspective can allow you to see a book in a new light, and a bit of life experience can enable you to edit it/write it better than before.
Then I realised if you rewrite something too often, sometimes the light goes out of it.
Then I realised, if the story is good enough, it's good enough.
Ahem.
At the moment I'm rewriting two stories that are both more than five years old in origin. Both I'd given up on in the past, but however often I've tried to forget them, I've never been able to fully shake them out of my brain. A reread suggests to me there's something in them worth exploring.
One of them I retyped into a blank word document ie forcing myself to type each word out fresh. That way I wasn't just editing the old story, I was both revising each and every word and also getting back into the world and the feel of it all. It's helped a lot.
Can't say either will ever get published, but I need to get them out of my head.