In at least two places where I worked, either the consensus or outright explicit policy was that you cannot expect reply to an email earlier than 24 hours after

In at least two places where I worked, either the consensus or outright explicit policy was that you cannot expect reply to an email earlier than 24 hours after sending (that is, you are not expected to whine or escalate or send another email about the same issue.) On the other hand, there was an expectation that if you receive an email and cannot do what is requested in the timeframe specified in the email or within 24 hours if not specified, you're expected to send SOME reply - decline the request, ask for more time, recommend another path etc.) Overall, SLA for stuff requested by email was "next business day" :) anything more urgent should be done by instant messenger, phone, walk-in and such.