Drip and email campaigns typically have optimal results every 3 to 4 weeks to nurture prospects through a sales funnel (direct response campaign management). It is as much the frequency as the content and messages that is provided to help the prospect convert. In some cases, frequency as much as daily is used (groupon), while in other services or products (industrial) monthly frequency is usually sufficient. Monitor your open and conversion rates using applications like Mailchimp to optimize your campaign. Let me know if you need assistance/clarification.