I frequently visit a couple of websites which support on my favourite sport. Both produce newsletters which are generated by events. If there’s nothing to report then a week can go by between editions. Following a match or a bit of news, I can have a dozen fall into my inbox during the day. Nirvana!
Both sites offer, as an alternative, the option of a weekly compendium. At the start of the season they both send newsletters to ask if I wish to remain on their list together with other options, including the offer of the weekly newsletter for the less than nerdish.
You might want to consider something along the same lines for the subscribers to your email marketing list. You probably already ask them for their preferences when split testing is inappropriate, such as the means of address. Many companies ask on sign up.
Sending time of a marketing email is a different proposition.
You will have data defining the best sending time for certain groups of subscribers. For instance, you might have found that position in the company predicts the time that gives the highest open rate. Those whose function requires them to be out and about might want a different time to those who are office bound. However, job titles can be confusing and often do not define the role.
Try asking a statistically significant but small section of a particular group what their preferred time of reception is. Whatever the reply you will learn something useful.
For instance, if the replies generally conform to that of the group as a whole then you can be reassured. You have split your list well.
If, however, there is a great variation then you know that you need to work on the way you discriminate between subscribers.
If there are two or three distinct answers then you should discover what is common amongst those who pick each particular time frame.
There is a restriction to consider. Do not offer a free choice of times as you do not want to be sending them off every five minutes. Pick those windows that you are happy to service as now that these people have indicated a preference, you must conform to it.