from a personal perspective I would say once a week in order to remind your customer or your potential consumer of your existence and your products or services. from a business perspective, the key word here is 'IT DEPENDS" do not wait for a given and exact frequency. you should take your kind of services or goods you have into consideration, as a simple example if you have a store that sells fruits and vegetables or fresh goods in general you can email your customer every day or one yes one no sending the menu and what you have for the day or the week. if you have a clothing store you can email them every time you have new collection, sales, coupons, and special offers. so, here we can return to the key word it depends on the need of our business.