Depends on the industry. I’ve always looked upon it as:
20% of your clients will be needy at any given time, while
80% are on autopilot. If those 20% are always the same, time to get rid of them...
That’s another variation on the same theme. The 80-20 rule applies if you are only dividing your group into two parts; the 80-15-5 rule works better if there groups are more appropriate.