This has got to be the number one question we get, both from direct customers as well as our SEO reseller team. it's natural. You are buying a product designed to help your search engine rankings. So you want to know how long it will take to work.
I imagine that doctors get this question constantly. "How long until that medicine takes effect?" And doctors are probably always somewhat worried about the answer as the human body is an extremely complex mechanism. Given that there are so many factors at play, the effectiveness of any particular drug will be different on different people. Well, this is a great analogy for the SEO reseller who is trying to find the words to help their client understand the SEO process. Each website is different. It has it's own distinct history with Google and the other search engines. Likewise, each keyword space is very different. Some are extremely competitive, while others are not.
In sum, the medical analogy is a helpful one for SEO resellers. But if that doesn't help your anxious client, you should also point out that Google very specifically tells the SEO community not to promise page 1 rankings. In fact, in their guidelines on how to hire a good SEO firm, they specifically mention staying away from anyone who does make that promise. I've blogged before about this and the problems and subtleties with SEO guarantees before. This is a complex issue.
Sorry - Was interrupted by a phone call from a potential customer asking "how long until we rank?"