I’ve always made a habit of responding to email messages within a day (even if just to say I’m sorry but I’m busy right now and will try to get back to you next week–or something like that). While I don’t always live up to this standard, I believe it is common courtesy to acknowledge that someone is trying to communicate with me (unless they are simply trying to sell me something).
While I consider responding to emails in a timely fashion the professional thing to do, I recognize that some of the people I am trying to reach are overwhelmed and simply don’t have the bandwidth to respond.
Many lawyers make effective use of auto replies