The standard auto-reply (they call it vacation responder) in GMail is a little weak. It's either on or off. You need to remember to set it right before you go on vacation or wherever you are going and you need to remember to turn it off when you get back.

Google labs does offer the ability to spice this up though. Using their experimental functionality you can set the date on which the auto-reply will turn on and off. That way you can set it ahead of time and forget about it. It's one less thing to worry about right when you are leaving.

To set this up go into Gmail and click the "Settings" link (upper right of the page). Once you are in Settings, click the "Labs" tab. Scroll down to the "Vacation Time!" project and select the radio button to enable this.

Once it's enabled you can set your auto-reply in the usual fashion by go to "Settings", selecting the "General" tab and scrolling down to the "Vacation responder" section. Once the labs feature is turned on this will no longer be a set of radio buttons. It'll be radio buttons with dates included like so:

The GMail vacation responder settings when the google labs vacation time! feature is enabled.