In the crowded world we are living in today, I am not sure if this rule is still valid. It is a part of a marketing job to send messages to customers and to expect them to take actions. For a direct response marketing, it’s getting more and more expensive because you send a new message to a new group of customers all the time. A branding marketing is better in this aspect but this approach takes longer times with consistent focus.
The rule of 7 says that a customer has to see an advertisement or message 7 times to remember it or to be inspired to take action. The rule applies to most if not all channels e.g. TV., emails, social media.