A question that I am often asked is, "How often should I send my email newsletters?" and of course the answer is the same, "well, it depends", but there are a number of questions that should be asked first:

How long does it take you to develop fresh new content?
What resources are available to you?
How often do you do direct mail?

If you are truely doing a newsletter, monthly is a good starting point, if you can get in a rythem and the newsletters are coming easily to you, try more frequently.

If you notice that your response rates - open and click - are declining recently, take a step back and slow it down a bit. This normally helps your response rates.

Start with an A/B split, mail to one once a month and the other once a quarter, see how the results differ, it is likely that that once a quarter email statistics wil be lowers, but this is not always the case. The other thing to consider if you are looking at emailing less than once a month is the loss of mindshare among your subscibers.