For me, there isn’t one. I think the social contract of email response time for random inquiries is maybe someday. I might just wait months if it takes me that long to have interest in what you are saying. Or you might get a reply in 30 seconds if I’m stoked to hear from you.
Of course that changes based on the relationship dynamic between the sender and receiver. An email from your co-worker or boss asking you a question has a different social contract, and the only way to know what it is is experience with those people.