Dream. You're driving on the freeway. You are behind the wheel of a car with unlimited power and speed. There's no speed limit and nobody's on the road. You decide to test the limit of the machine you're in. How fast can you go? You progressively shift into high gear and push the pedal down. You gain speed quickly. It's exhilarating. You push it a little more, reaching a velocity you've never experienced before. It feels great, and you feel powerful, as if your wingless machine was about ready to take off and fly into the horizon. The cockpit is shaking a little, but that's all fine: you know it can take it. So you push the pedal a little further down, with an eye on the speedometer. How fast can you go? How much can the engine take before you loose control of the machine and crash? How much are you willing to risk to go just a little bit faster?
Email is very much like a freeway. In theory, there is no speed limit. No one to stop you. No hard stop on how many emails you can send. Yet, like anything that is free, email is often abused, so in practice there are boundaries. You can't email anyone and everyone, and you can't flood your subscribers. You have to keep it all under control to avoid complaints and blacklisting. Yet you want to maximize your investment. So the question is: how much is too much?
You could test your frequency. It goes like this:
If opt-outs and complaints go up, but conversions also go up, your frequency test is still a winner. [...] If opt-outs and complaints go up, and conversions stay flat or drop, your frequency test is a loser.
However, that approach assumes a linear progression that is independent of any factor that might otherwise influence the results, one way or another. The reality of communicating with your customers or subscribers is unlikely to follow a straight line. Losing recipients that have expressed an interest in your material is a cost that will have long term effects on your business. That lost may well offset and ultimately dwarf any short-term profit you can benefit from. Maximizing the short-term profits may not be as wise as it seems, if it means trimming your subscribers' base every time you run a new campaign. It'd be akin to fishing with dynamite. There may not be any fish left in the pond the next time around.
The best -- but more difficult -- approach to managing email marketing campaigns is to vary your send frequency based on the response rate, the quality of the material you are serving, the uniqueness of your offer, and finally the market fit of your product relative to the demographic. To be fair, balancing these criteria is much more of an art than it is mathematics. It's an art which, by definition, you should already have a good grasp on. Like all art, you need to keep working on it. Test everything, and do your best for your customers and recipients.