My guidelines:<p>If your email is short, very clear, acknowledges that the recipient is likely to be busy, and only requires a short answer, usually you will be answered with 30 minutes of it being read, or never.<p>If it's long, unclear, rambling and makes no effort to demonstrate that you are in effect asking a favor, or if the reply requires thought and detail, then a week, or never.<p>Ask yourself: what have I done to motivate the person to answer this email?<p>Also, read this:<p><a href="http://linuxmafia.com/faq/Essays/smart-questions.html" rel="nofollow">http://linuxmafia.com/faq/Essays/smart-questions.html</a><p>Don't take all of it literally - pick, choose, and use common sense.