Re: Platform building: when do you mention your upcoming work?
For fiction, I would only wait until you know you have a work to announce and promote. That is, it has a beginning, middle and end. So maybe that's when your draft is complete, but it could come when your outline is complete and you know in your gut that you'll finish it.
The only thing you want to avoid is to have a fan ask you "whatever happened to Project X?" and you have to say "Oh, it wasn't going anywhere so I just stopped writing." Though it's always better to say "Oh, this fabulous other project that I already have a contract for took over, but I hope to return to Project X someday."
Before you get to the "announcement," you can always say you're in the middle of a project without defining that project.
One more piece of advice: If you're unpublished, do not wait to set up your social media presence. Get yourself known as a writer, and an interesting one at that, and your project, when complete, will get noticed.