Unless you have a publisher calling the shots, when your book “launches” is up to you. In my opinion, the ideal launch time is as soon as the book is finished. No one can buy it if it’s not available, and you can’t make any money if no one can buy it, right?
The marketing efforts surrounding your book can start long before the actual launch date. For example, if you plan to do any media outreach, many publications (especially print) won’t review books unless they receive them several months ahead of time. Depending on the time of year, you can pitch your book for inclusion in various “seasonal” articles, e.g. holiday gift ideas for kids, beach reads for spring break/summer, etc.
Digital publications are more flexible, but you’ll have a better chance of getting a “yes” response if you’re able to offer a review copy (even a PDF or digital file) before your book comes out. The cover doesn’t have to be finalized and the book doesn’t have to have had a final proofread for the press to accept what’s called an “advance reader’s copy/ARC” or “galley” copy.
Even if traditional media outreach isn’t part of your marketing plan, you can (and should) offer early copies to book bloggers to review. If and when a blogger gives your book a positive review, you can use that for additional marketing purposes, e.g. highlight it on your website, your Amazon page, or when reaching out to your college alumni publications, regional alumni club newsletter, etc.
Marketing is all about getting your book in front of potential readers. You never want to rush and risk compromising quality, but once you have a (nearly) finished product, I say get started on that front. Then when the final version is ready for public consumption, go for the launch!