For most “local” forms of advertising, like online, newspaper, radio and television, I recommend you invest for at least three months and preferably six.
Online advertising has an extra special benefit. While you can create loose ties between radio, print and television advertising and customer response, like asking people how they heard about “X,” it’s an indirect science.
Most online advertising options — like Google, Facebook, Twitter, LinkedIn and Pinterest — have built-in reporting. These simple reports can show you how many people saw your ad (in theory) and how many people clicked on it. If you’re smart about directing each type of online advertising to a specific promotion or page on your website, you can match closed sales with the ad campaign. You can’t really “assign” these sales to the ad, but the process will give you an indication of which ads people are responding to.
As you build online advertising sophistication, you can start to run multiple versions of advertisements within any given online network. For example, consider creating three different flavors of the same ad with Facebook or Google Adwords. The services will allow you to display randomly one of them to any given viewer. As time goes on, you can easily see which versions perform best and optimize based on results.