I've had such a manager. Her policy was that, during working hours, all staff should reply to managers' emails within an hour, if for no other reason than to acknowledge receipt and promise a lengthier response later. Staff would respond to external emails or colleagues' emails by the close of play; as with managers, it was acceptable to reply briefly to acknowledge receipt and return a real answer the next day. Managers, for their part, responded to the rest of staff within an hour as well. She was firm, but this worked out very well for all of us, and I continue following her policy in my new job elsewhere. My new boss loves me for it, because he knows I've at least read whatever he's requested.