What if they are just opening them but they are not clicking on them? How long do we keep them if that is what is happening?
I would keep all emails that are opening for as long as they continue to open. That shows they are at least interested in receiving messages from you.
If they are not opening you need to test and figure what will get them to. At least you have the ability to test because they are opening.
If I have users that are on my list for any length of time and haven't converted. I will send them an unsubscribe request and ask if they are no longer interested in X to unsubscribe.
I will send a message and ask if they are still interested in X.
I will send a message and ask what's holding them back.
Obviously, you can also promote other offers.
You could poll them and see if their interests have changed.