There's really no hard and fast answer to this question, but I can tell you that most people are so afraid of contacting their subscribers too often and annoying them that their err on the side of caution and don't contact their people often enough.

You need to remember that if you're not constantly reminding people that you're still there, that someone else will, and that person will likely forget about you.

For example, if you're not contacting your subscribers at least once a month, you're just not contacting your customers often enough. A lot of people think that contacting subscribers everyday is way too often, but there are certainly areas where it works. I am on a list right now where every weekday I'm sent a short little email, and I read it everyday. However, these aren't marketing and sales pitches, but rather little motivational blurbs – they're short and take me less than a minute to read.

A good rule of thumb that works for a lot of people is to contact your subscribers once every 2-5 days. That's often enough that they'll remember who you are but not so often that they're going to think they're being bombarded with messages. Feel free to play around with this number and see what works best for you and your subscribers.

Also remember that not every contact that you make with your customers needs to be an advertisement for something. While sending free content to your subscribers or past customers may seem like a bad financial move, it can be a way to build up good will with your past customers or current subscribers, and when it comes time to buy, they're going to remember you, and will be far more likely to do business with you.