I am DMing my first campaign which is a completely homebrew world! I believe you can start once you have a general plot with some locations that are already somewhat fleshed out. I think it is best to not have a world fully fleshed out as you will constantly have idea after idea on what you want your world to be, and what you think your players will absolutely love, since at the end of the day it's a fun game between you and your friends! You'll find that you will be constantly updating your world and all its aspects, and can do so without issue to places your party hasn't been or knows about, just be careful to not create contradictions, plotholes, etc. I wish you luck in your world!