The way you do it (or even if it's possible to do it at all) will depend upon how you get your emails. Do you use an email client on your PC (such as Outlook Express, Windows Live Mail or Thunderbird) or a web-based service (such as Gmail, Hotmail/Outlook, Yahoo, GMX, etc)? Also, exactly which service do you use? (i.e. what comes after the @ sign in your email address?)
>>> took 7 minutes for the 'automated' reply to appear
People using an email client (such as Outlook) can configure how often that client looks for new emails. If, say, the client checks for new mail every 10 minutes, then it would be normal for someone sending mail to that address to have to wait for up to 10 minutes to get an 'out of office' message back. (i.e. the client can't send out a response until it's recognised that there's incoming mail and it's only looking for it at 10-minute intervals).
The funniest one was when Swansea council emailed the translation department of the Welsh equivalent of a road sign. They received an out-of -office response in Welsh which they took as a translation and duly posted it on the signs.