With email marketing, personally I would say email every week if you are a e-commerce site and have physical products to sell and not just services.
The emails must be of quality, and should ideally be tested to cater for online email collection, and well as better know programmes that collect on the PC via a pop3 or similar method. Online mail boxes often if not always strip out custom formatting, so you need to either use in-line CSS or HTML format as you go along. A badly formatted email will also cause an unsubscribe as well as bad content.
If possible, try to pull in the email address you are sending the email to, this will show if a person is receiving 2 or more emails, what email address it is coming from, and also show them, you are not sending duplicate emails to the same address, but different subscribed addresses. An unsubscribe feature is also advised.
With email content, special "Email Only" offer's can often work well. If the customer only gets an offer because they are subscribed to your emails, this can help keep the subscribed numbers on the rise, rather than dropping. The content must be fresh, and sue images for products where possible as the images generally sell the product. You can even run simple cometitions as well and try get some interaction between yourself and the customer. Above all, make the customer feel wanted, and not abused, if possible use their name at the start of the email "Dear Mr J Smith" If you speak to them by name, they tend to listen more than, Dear Customer, cause everyone is a customer, but not everyone is a Mr J Smith.
Also test your emails. Test the response rates on days, open rates on times, and segment your data so you can send different offers or content to different people, i.e. send customers who have brought in the past 6 months a Free Delivery offer off, those 6 - 12 months a 10% Offer, Over 12months, send them a 15% offer and try to reactivate the customer again. Emails are a great way to speak to all your customers new and old. Times and days will make a massive difference, so test, and track stats weekly. If you cant track the stats let me know and I knwo of a good email service you can take a look at that will help you improve your email campaigns.
Good Luck
Lee