Wouldn't it be nice if there was a number or a schedule you could follow for how often you should email your list? While it would be great if there was research that suggested that mailing exactly every 5 days gets you the best results every time, there is no such thing. And there's a very good reason for it.
Every market, every niche, every audience and every person is different. While you'll never make everyone on your list happy, there is a lot you can do to make just about any email schedule work.
Stay top of mind
Let's lay the ground work first. You don't want to have too much time in between emails, or your readers will forget you. Anything less than once a month is not a good idea. In most markets and for most business models you should email less than twice a month.
On the other end of the spectrum, you don't want to go any higher than one email per day on average. Yes, you may have days when you have a good reason to send multiple emails, but on a weekly or bi-weekly average, you don't want to email more than once a day.
What is your goal for the email list?
Start by looking at what you're doing now. Then figure out how often you want to email. Do you want to grow a closer connection with your market by emailing more often? Do you want to drive more traffic back to your site by emailing them links frequently? Do you want to grow your income by making more frequent email offers?
Once you know your goal, you can start to make a plan for getting from point A to point B. What you don't want to do is to go straight from emailing once every few months to daily emails. It'll get your readers clicking the spam button like crazy. Instead, start with with a reengagement campaign that sends a few emails over a 1-2 week timeframe, then let your readers know you have more to share with them and start mailing weekly. Then if your analytics are good and you feel your list can tolerate it, ramp it up to daily emails.
You can also create an interesting reason to email your list daily. For example, while you usually publish a weekly newsletter with the occasional promotional email in between, running a 15 or 30 day challenge for your readers is a great excuse to hit their inbox daily without seeming pushy or spammy.
Listen to your audience when you get feedback on email frequency but also realize that there will always be someone complaining. Look at data like open rates to get a better feel for what frequency is working best for you and your audience.