Many times I've had people ask me what the right frequency of mailing is.
I've always said that the right frequency ends up being marketer discretion. In almost any case, sending more mail will make you more money. At least in the short run. At some point it seems like too much and people "feel like a spammer". That is generally the cutoff point. (You could try to run a rigorous test of unsubscribe and LTV, but I see few willing to do that.)
Seth Godin has a nice post on the topic today. See