I call it the *1/5 rule*.
It simply means that for every five posts you create, one can (and should) be a promotional call to action (CTA) for one of your services or products. There are exceptions to this rule ( of course), i.e. you are a retail store selling clothing etc.
People usually don’t feel that it is too excessive when you promote your products/service once a week on average.
In order to hit the 1/5 rule I find it easiest to use a calendar that I design for my clients withthe different pillars of content. Then write up the content in batches before posting. That way you dont fall into the trap of “what should I post about??!!”. Its clearly measured, thought about and not repetitive.