The inevitable question asked by every client (and just about anyone we meet on the street) is:
“How long will it take for my site to start getting free visibility in the search engines like Google and Yahoo?”
Every SEO has their own answer to this question. On a brand new site, it will come to considering the strength of the competition vs. the number (and value) of inbound links to your site. We have seen sites rank for fairly valuable keywords in a matter of 3 days, and we have also waited months for other sites to start ranking regularly on desired keywords.
Matt Cutts answered a video question about this very topic.