I generally respond quickly, as soon as I get the email. While there is something to be said about people maybe subconsciously valuing your time more if you wait, I've found that replying quickly has earned me a reputation of being well liked, reliable, and prompt. The times I don't get back to people quickly they tend to assume (correctly) that I must be busy or otherwise occupied.

While this may be attributed to the social climate of the jobs I've held, I believe that if you work with genuine/empathetic/normal people this is a good strategy for someone who has the natural inclination to read and reply to things immediately. If you're working with narcissists or people who don't value anyone's time but their own, definitely set up strict rules for yourself (e.g. only checking email once a day, first thing in the morning).