The time to ask for advice is usually after somebody has pointed out a problem.
I often bring this up in other threads, but a common thing in criticism is that readers are good at noticing problems like: "This feels really slow", "That chapter was a bit boring", "I don't care about this character", "That plot point came out of nowhere." but they're not great at giving good solutions to those problems. For solutions, it's good to ask an expert.
(As an aside, sometimes nobody outright points out the problem, sometimes you have to work it out from clues, like how reader comments all seem to interpret a character or plot point in a way you didn't intend, or the big important dramatic chapter you thought everyone would go wild over gets no comments).
Sometimes you can skip the need to ask because you might find somebody's already put that guidance online or in a book.
Like these videos by Alexa Donne lay out a bunch of really common issues to look out for, and how to fix them:
Or you might find books like "Save the Cat! Writes a Novel", "Romancing the Beat" or "Steven King: On Writing" give great guidance on writing and save you needing to seek a mentor, or at least give you some context to help you ask for the specific advice you need.