This will depend on your business and your current marketing efforts, although a general rule to follow is at least once per month, if not once per week. Small businesses with less than 2,000 contacts should send one to two emails per week, but larger corporations may need to send out more to keep up with their larger contact list. Even if a customer is not ready to convert yet, sending frequent emails is a great email practice to have to remind them that your business still exists. That way when they are ready to convert, you will be the first one to pop into their mind.