Most people tend to do ongoing works here.
I think if I was going to do a novel, I'd finish it first, so that I could then go and revise the beginning. I'd probably go through 2 or 3 full drafts, and then publish the chapters on some schedule. You'll definitely want to start advertising shortly before it comes out though.
It's a good practice when working creatively to tell nobody about what you're doing until you're just about done. Telling others just lets the energy out of it! Think of the many people who tell people they're working on their novel for years, but they've only written 1.5 chapters at the end of 5 years.
My comics take too long to finish the whole story before coming out, but I had 72 complete pages (3 chapters) before I started posting them publicly online. Most webcomic artists don't build that kind of a buffer, and would consider that excessive.
Learning to advertise without compromising your work schedule is just part of the gig for webcomic creators. It's easier if you already have established your production schedule as a habit so you can work around that. That's one benefit I had from building that large buffer prior to going live.
