I do month-to-month from Day 1. "If you stop paying, I'll stop working." I only work with one client per niche per city, so if someone leaves, I'll use their results to sell to their competition and pass them. Most other SEO's immediately screw up the SEO, so it's easy to pass them, as they fall like rocks, generally. I've had some clients for 10 years. Most clients have had other SEO's that failed, so they keep me just to "keep it ranking."