Technically, your web site is going to be publicly accessible as soon as you activate the WordPress software. However, that doesn't mean that you've actually "launched" it. In fact, even after creating your first web page, you're probably at least several days away from being ready for a true launch.

What you want to avoid is promoting your site before you have enough content. What's enough? That'll depend on the topic you've chosen. For some niche topics, 10 pages can cover a lot of ground. For others, you may need closer to 25 pages. You want to aim for a site that "feels" complete. A little wishy-washy yes, but you'll know what I mean when you get to that point. Note that a page doesn't include the archive pages which are simply lists of links and blurbs of actual content pages.

I highly recommend that you avoid creating a bunch of placeholder pages with no content or with under construction as the only copy. At all times your site should look complete even though you may have plans to add more and more content.

If you can get a friend to review your first pass at a web site, that would be good. Sometimes when you stare too long at something you can miss obvious deficiencies, whereas someone with a fresh perspective will notice them immediately. Also, you don't need to worry about having your advertising in place from the first day you launch. You won't have any traffic for a while so advertising wouldn't generate any money anyway.

You also want to make sure that your web site is good from a technical standpoint. That is, you should use the W3C's HTML validator or the FireFox plugin to confirm that your web pages' HTML is valid. Mistakes in this area can trip up search engines which in turn can negatively impact your web site's ability to get visitors from search engines.