[tobin]

A company I work for (an online parenting resource) sends out newsletters once a week to each of it's target markets. They also do a weekly "deals" newsletter which sends coupons to subscribers for deals they may be interested within their area.

The newsletters typically highlight events and new places to go in a specific city and editors choose up to 10 items to feature for the weekly mailing.

I think it really depends on your content. Some information may be valuable as daily mailings (closing market reports or summaries of the nights games for a sports league) or it can be less frequently. I guess It depends on how much content you want to be "important" to the user and how often.