The average touch rate for Email Marketing is twice a month. At the very least, make sure to send out at least 4 times a year. When people opt in to receive your emails, let them know how often you plan on sending so they know how much email to expect from you. According to Jupiter Research, when asked why recipients unsubscribed to opt-in emails 40% said they were getting too many offers. If you decided to switch from sending every 5 weeks to every 3 weeks, monitor your opt-outs to see how your audience is reacting.
Additionally, Constant Contact offers a Planning for Email Marketing webinar for free that you can register to attend here: http://www.constantcontact.com/learning-center/webinars/live/details/Planning-Email-Marketing.jsp