SEO is such a big topic that it's hard to classify with one brush stroke.
A lot depends on the condition of the site before optimized. Was it awful horrible rotten and now magically wonderful? (The cinderella effect) Changes could be seen almost immediatly depending on your stated (if any) revisit cycle.
Was the site a spam ridden slop site and now fixed... those changes could take longer as your pages may have been sent to supplemental or otherwise penalized.
Was the site semi okay? if so the on page
seo may not make all that a startling effect. Alot of
SEO benefits come with Age......more indexing, broader indexing, (read IBL). Doman Age and other factors apply too.
Finally On page
SEO will have little to no results if the content is not apetizing to the spiders. Links pages, virtually empty pages, pages off topic can all be a problem.
So the answer to your question is a definate.....it depends.
pPace your site on the review my site page and gets lots of free advice. Just remember free advice is only worth what you pay for it.....sometimes