When is a book ready to publish?

I recently had my first public speaking opportunities (indie published two books but just dipping my toes into marketing), and both times I was asked how to know when your book is ready to publish. I’ve just gone by my own intuition… which is probably not the best method.

What I told the attendees was that you have two choices: either to ask whether your book is something your target audience is willing to pay money to read, or to ask an industry professional (editor, another author, etc) if it’s ready. It was hard to give good advice because my recommendations would be very different for an over-confident writer and a self-critical writer… and I didn’t know where these people were at.

Anyway, does anyone have any more thoughts on that? Any succinct answer that would be helpful to both over-confident and self-critical writers?