A simple solution is a time-delay relay (or multi-function timer) like this one:
http://www.alliedelec.com/search/productdetail.aspx?SKU=8190006
(I haven't used that one, it's just the first one that came up in a search. I have used similar products.)
You simply turn on and off the signal to it faster than the timer setting. If it doesn't see a pulse within the set time period, it turns off. Make sure that your code actually cycles the output on and off, don't use some sort of pulse train generator that could keep running in the event of a software crash. You may want to connect this to a latching relay so that the operator needs to push a button to re-enable it after re-starting the software.
