From Wikipedia we have "The Pareto principle (also known as the 80-20 rule, the law of the vital few and the principle of factor sparsity) states that, for many events, 80% of the effects come from 20% of the causes."
(See http://en.wikipedia.org/wiki/Pareto_principle for more.)
Applied to learning music we get this, 80% of the benefit of a practice session is gained from 20% of the effort. Could this be true?
When I get started in the morning I usually play over a tune slowly and carefully. I pay attention to how my notes are tuning in to the open strings. i listen to the sympathetic resonance. This really warms up my ear as well as my fingers and arm.
Playing the tune through the second time is easier. If I play the tune five times it's beginning to sound solid by the end. If the second time through sounds 80% as good as the fifth time, then we have an example of the 80/20 rule.
Until this relationship struck my mind, I had not been aware that practicing by fives lets you employ the Pareto Principle.
Another way to look at it is to ask how much can you gain by additional practice.
If you typically practice about a half an hour. then you could enhance your ability 25% by continuing two more hours. (The half hour is 20% of two and a half. The marginal increase in results of 100% over 80% is 25%.)
How well I play, or how well you play is subjective for us. Even so, I suspect there is at least some truth to the application of the 80/20 rule in music practicing.
One last thought. It may be the number of times you play the same passage, piece or etude. That may be the constraint that brings the 80/20 rule into play.
This concept has a little more detail, especially for music students, at the Music Lesson Plan blog.