Email Fatigue Measurement

Does anyone have a recommendation for a good way of measuring email fatigue? We are trying to look at the count of emails a person has received over a set amount of time and whether that has any impact on their engagement levels. I think the next level of this would be to see if there is an average amount of emails a person receives before they start to be engaged and move through the funnel. I have thought of a couple ways of doing it, but neither of those ways seem very reasonable or accurate, so I'm curious if anyone else out there has come up with a way to measure this.