It's really inconsiderate. Unless I am physically more than a mile from my laptop, I can probably reply to your e-mail within a minute and a half.
I expect the same courtesy in return. This whole "wait two days and then reply" thing is totally unacceptable. Maybe if you have some complex response to formulate that involves getting a large number of ducks in a row, but, other than that....
It never helps to have e-mails hang around in one's inbox. Once they're there for a couple days, I just forget I have to answer them. So the whole deliberation thing kind of backfires.
I can't believe people aren't always on tenterhooks to see if I've e-mailed them.