HAPPY NEW YEAR!!
I often get asked, "When should I get my cover design?", and the answer, in short, is: It's up to you...
You must have your cover designed before you publish (obviously) but don't leave it too late. If you have a set deadline for publishing your book, make sure you have time to not only have your cover designed, but also to allow a window of time for any edits or adjustments needed.
Some of our clients have their covers designed during the early stages of writing, often somewhere in the midst of thrashing out the first draft. This helps the author draw inspiration seeing the cover as a representation of their finished work. If you are publishing in ebook format only, this is perfectly fine.
If you are wanting to publish in paperback format there are other things to consider, most importantly, the page count. It is likely your page count may change, quite dramatically in some instances, between your first and final draft and between your final draft and editing. You book's page count determines the width of the spine and therefor the size of the cover design. And so the paperback cover design can only be finalised once the final page count has been determined.
But don't worry: If you do want a design early on in the process to help motivate you, I'd recommend creating the ebook cover only and getting your designer to add the spine and back when you have your final page count.
Make sure you communicate with your designer so they understand that you want to adapt the ebook into a paperback so they don't close off any possibility of expanding the design or deleting your "finished" file before they can work on the paperback design.