Getting feedback on your work can be helpful at any stage of the writing process (even if you haven’t written anything yet and are just kicking around an idea). Whether or not you’ve finished your manuscript matters little when seeking advice on how to improve it.
In fact, getting advice on both your content (is your main character too drab?) and your writing (is your voice too passive?) in your draft’s early stages could keep you from laying a poor foundation for the rest of your book. This sort of preliminary critique could help you avoid wasting time and energy on material that isn’t working, and steer you in the direction of what will work.
What does matter when it comes to critiques is not how far along you are, but the source of the feedback. Your challenge is to find a person who can offer up a valuable, worthwhile assessment of your work that can help you inch your manuscript closer to publication. That means, for example, that if you’re writing a young adult novel, you’d be best served by a skilled reader with experience in the YA genre.
If you’re willing to invest money in improving your project, you might opt to hire a professional editor. But you might also find trusted readers by joining a writing group or connecting with a skilled writer happy to serve as a critique partner (online or off). The cost here is time—your time, as you will need to critique work in return.
Regardless of the path you choose, just be sure to gauge the value of the advice being offered and exercise your best judgment on what to use and what to quietly dismiss—especially when your project is still in the formative stages.