Wouldn’t it be nice if there was a number or a schedule you could follow? 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 frequency work.