Sending emails too frequently will motivate your subscribers to end their subscription. Sending emails infrequently, and you’ll lose rapport. There’s not an optimum schedule for sending marketing emails, because each email campaign has a unique purpose. That said, I would recommend not sending emails more frequently than once per week, and not less frequently than once per month.
If you find that setting aside time in your schedule is difficult, I’m with you. Here’s a trick. Block off a few hours on a particular morning each month. Write your next two or three emails, and stagger the delivery of the emails to meet your strategy – with most email marketing services (but especially with Online Outbox), you could schedule Email #1 to be sent today, Email #2 to be sent next Tuesday at 8:00am, and so on.