This depends on the goals for your email, the type of content you are sending and, ideally, the preferences of your email recipients.

General Guidelines:

• Mail at least once a month. Mail less often than this, and you risk being forgotten by recipients. Monthly is the bare minimum if you want to keep your brand or company name top of mind (a common email goal).
• Let content be your guide. Look at what you provide readers and you’ll get a feel for proper frequency. Analyze how often the information changes, and how quickly readers must receive it to act on it.
• Take the lead from your readers. Some organizations offer daily email newsletters as well as weekly summaries of the same content to give readers a choice. Always tell potential readers how often you mail at sign-up, so they can decide if the frequency works for them.
• Work within your resources. A daily email requires far more resources than a monthly. Better a well-done monthly email than a shoddy weekly or daily. I usually recommend clients to start with a monthly. Once that’s going smoothly, they can think about moving to weekly. You need to walk before you can run!
• Watch for trends. Declining response, open, and click-through rates can be signs of list fatigue. Though some decrease is normal, watch carefully and cut back frequency if you see a problem. Don’t assume if the unsubscribe rate is stable you’re OK. Many people prefer to forward email directly to their delete folder rather than to unsubscribe.

These guidelines come from an article by Jeanne Jennings, who offers other email guidelines, including six steps to controlling your email list. http://www.clickz.com/3489391