I tried to set this up with Simple Automation Rules right now, but couldn't find a way to delay the "off" action based on the "on." I don't use SAR (formerly Simple Lighting, which it may still be called if you're running old hub firmware) much, so I might be missing something.
I did write an app that should work for this: [RELEASE] Timed Switch Helper. My intent was actually to use two different switches (in my case, something like a "timed fan" virtual switch that would turn on the real fan switch for a certain amount of time, then turn it off), but I don't see why it wouldn't work for the same purpose that you're aiming for if you just choose the same switch for both the "virtual"/timed and "real" switches. Ignore the rule I wrote in that post--it's from Rule 3.0 and would be quite different in 4.0, which is what you'd have if you just installed RM.
