With my work in the not-for-profit realm, I have found that the perceived value is similar to the for-profit realm: frequent communication is appreciated if there is valuable content.
An easy way to see if your list is being burnt out is to run A-B testing, run promotions asking about prior content, to see if your recipients are reading/retaining, and to simply ask your recipients if they feel bombarded.
“David offers an incredible amount of diversity in developing marketing and sales programs, and has the hallmark of great managers - he asks questions and probes before devising a winning strategy.”
~ Business Manager, Miele 
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