In the technical sense of the word, an ezine is a regularly scheduled follow up email message, which is sent to a group of 'opt-in' readers, who wants to know about your specific niche topic. This means you send it out much like a magazine would be sent – on a set scheduled basis. If you were to base it off most magazine schedules, though, you would only send out an email once a month. At most, you would send an email once per week, just like some of today's magazines are published.
However, you are better served to think about an ezine as any sort of periodic follow-up with your readers, who are interested in you niche topic. You can follow-up with them – every day, every 2-5 days, once per week, or even often you decide work best for your strategies.
When you think of an ezine this way … YOU decide, along with input from your readers, what is the best email schedule for putting out new content. Because a magazine is published on a consistent basis, it is a predictable form of publication that many readers like. However, it could have better for you to publish content more often than once a week. Even if it means sending messages before the next one is scheduled. If your readers tell you (or respond better) to a regular publishing schedule, then you'll need to decide if you want to match their needs, or make your own publishing schedule.
The great part about ezine publishing is … it is up to you, as the publisher, to decide when you want to publish.
In my experience … I have started with a one to three day introductory training schedule, and then as that beginning training concludes, I revise it to a consistent two to seven day publishing schedule. Monthly is WAY too far separated to stay on the top of your reader's mind – in my opinion. You want to always be there, always producing great, cutting-edge information that helps your reader move to the next level.
And you do not want them to ever forget that you are the one coming up with the top-notch content.