Found an interesting mini-case study over at Tamara’s blog about how one marketer cut down on the number of emails it sent (some of their subscribers got up to 10 emails a week) to about 2 emails per week. They don’t mention where conversions were before the change, but now they get a 10% conversion rate.
Reminds me of an Open Rate example I like to show people. We watched a customer send email newsletters over the course of 2 years. You can see his open rate drop over time as they upped their frequency to twice a month, then to twice a week (click to zoom):
Look at that open rate drop. There are a lot of other factors that contribute to a declining open rate, mainly "Was the content any good?" But the main takeaway for me is: "If you’ve got nothing useful to say, don’t send anything."