Reorganized from the original conversation at "Question closed" notifications experiment results and graduation (thank you for the question. The informative discussion felt supportive to read and explained it was not just me), where it was cited that:
"... Additionally, the system default interval to receive an email notification for unread inbox messages is 3 hours. —copied http://web.archive.org/web/*/https://meta.stackexchange.com/questions/356742/question-closed-notifications-experiment-results-and-graduation from https://meta.stackexchange.com/a/356767/163204 & "Question closed" notifications experiment results and graduation where I originally commented, thank you https://meta.stackexchange.com/users/282094/rob for the encouragement, and then https://meta.stackexchange.com/users/284336/catija for backup.
Why "3 hours"? Is Stackcorp's bulk/mass emailing really the issue (server stress and spam complaints basically?)? I want instant and every notifcation, personally*. Can I please pay $5/mo. or buy credits for instant and you pay for longer timing if you prefer? Picking "three" hours for everyone is a lack of thought.
Programmatically a customizable form to allow the user to set the setting is optimal, and would lead the way as a role model for other websites suffering with the same systemic/formulaic poverty of thought.
*I can not trust the current Notification emails to be regular, I manually refresh SE.com constantly just to stay informed (clicking the menubar's uninformative red light sequare, again and again), the ancient Android app is the most reliable/instant notification I get yet it's ethereal (restart phone and poof, missed several mods made in under 30 minutes, again), versus a copy of email apparently timed currently for dealing with server stress instead of user stress.