Shoutbox

Ah, you're gonna like this!

I found that Plus! has a built-in feature for displaying the number of remaining minutes until you're back.

Set your status to Away, then the Personalized Status box appears. Give it a name, then hover over the "Automatically reset my status" option and it gives you the tags you can put in the auto-responder, such as (!PS_RM) for remaining minutes.

Tick it, set a time, add the tag to the auto-responder and you're set!