According to searchenginewatch.com:
Honestly, the short answer is that it really all depends. Every company has its own distinct needs, with different customers, competitors, and individual sets of goals. If you’re trying to rank highly for hundreds of different keywords against heavy competition, you’ll need to spend far more time than someone who just wants a better chance at ranking locally.
In my experience, it takes a little more than 5 hours to come up with 4 SEO-optimized web pages, taking into consideration the length of the site description, content, keywords to be used, etc.