Why is the sales cycle important to SEO? It's simple: SEO is a long-term effort. If it takes six months to ramp up your SEO program, you have to wait that long, plus your sales cycle time, to be able to determine ROI. You can't kill your SEO program if you don't see immediate results; that's because it already takes time for your sales cycle to go from "new lead" to "closed."
If your typical sales cycle is 12 months, it could be 18 months before you could attribute that sale to your SEO.
Read more: http://www.marketingprofs.com/articles/2011/4838/the-top-three-b2b-seo-recommendations-to-consider#ixzz1JWnxCBVq