This is what some marketing experts call the "Law of 29." Whether or not you agree with the number of exposures it takes through the sales cycle is not important. What is important is that your prospect is going to have to see your message over and over before he/she buys your product or service.
Successful companies have the plan, strategy, and discipline to get in front of their prospects as many times as it takes. A typical plan is easy to conceive: postcards, personal letters, newsletters, calls, emails. The tough part is developing content and faithfully executing the plan.