At ATL, Brian Tannenbaum writes about lawyers who want to get out of the office. Brian says you should never reply to emails immediately–wait 12 hours.
1. I never respond to an email immediately unless it requires an immediate response. If you respond to everything immediately, you are only training people to think that you respond immediately. When they send an email at 8:30 a.m. and by 8:42 you haven’t responded, they will stalk you. Back off, respond within 12 hours.
If I can answer the email in a minute or less, I reply right away. If it takes a few minutes of thought, I leave it unread, and get to it when I can.