The rule of 7 is an idea in marketing that it can take hearing a message 7 times before we might take action.
Apparently the rule is from early movie marketing and while I don't know if I'd rely on the number it certainly seems a sensible idea to keep in mind that you can't expect people to take action the first time they hear your message — if you're trying to get people to try something new be it buying food, downloading your app, watching a movie, or a choosing an electric car, it takes a while and repeated exposure before you can expect it to pay off.
I can't remember where I heard it, but some advice I keep in mind when communicating is that by the time you're tired of saying it, others are just starting to hear it. It's not a bad thing to repeat your main message.
I learned about the Rule of 7 from the recent book Human Powered by Trenton Moss.
When putting an apple in the adverts on this sketch I was reminded that so often it's the things in the supermarket that are most healthy for us that don't get anyone shouting about them at all.