It took us nearly 17 years to recognize a fact about marketing that we already knew. I’m talking about frequency: how often you should “hit” your prospects with ads.

Instinct tells us it should be not too often. (You don’t want to burn them out.) And that instinct is what was behind the early formulas for sending ads to Internet prospects and buyers. The first rule was the most conservative: Only one email out of five could be a sales piece. Some of our bolder marketers did more. And before long, the rule was to send out seven emails a week, one a day. Four of them editorial; three of them advertising.

That became the dominant formula for at least a dozen years. And it was working fine. The problem, I knew at the time, was that it had never been tested. And then it was tested. And every time a test was done that “hit” prospects and customers more frequently, sales rose.

This is exactly what happened in the direct marketing industry 40 years ago. We went from mailing our customers twice a month to mailing them daily on a non-stop basis until the mailings failed.

A few months ago, I persuaded one of my clients to move from three emailings a week to every day. After two weeks, her sales were up 37%. I checked in with her yesterday to see if those results had diminished. “Far from it,” she said. “They’re up 47%!”

I understand the instinct to limit your advertising. Even after all these years of reality contradicting it, it’s still stuck in my brain.

So what I tell my clients is this: If your advertising is pushy and obnoxious, you will wear away your customer file with frequent emails. But if it is smart and informative and useful, you should increase the frequency until the sales report, not your instinct, tells you to stop.