After my site has been optimized and the key words have been defined does it require constant weekly or monthly updating , or only a few minor adjustments for a few months to increase ranking ?
I can tell you this: for sure most of the work is done in the begining. and then if you wish to maintain your position or advance your position you need to work at it. also because SEO reality of real life and technoligies change with time. I guess the 20/80 principle applies here too. ie in the begining you have to to about 80 percent of the work and the on-going continuing maintainance is the 20%
You must ALWAYS be applying SEO techniques. Blog entries and comments, articles, press release, forums and other link building and always update the site for new or fresh content.
I agree with the 80 20 theory. You dont have to work all day and write press releases, but you do have to see which pages are still not strong enough and keep improving.
You will need to keep your link strenght and quality the same, pages move, links will move from a front page, pages will be archive and loose link juice, so you need to keep working at it.
Constant work... I agree with the 20/80 comment. I'm still in the 80 and working hard to maintain positions for various keywords. Whenever I don't feel like working on SEO, I just ask myself how many others in the same market are doing it RIGHT now. The answer is a lot of them.