Except for maybe the early 6.9s no such thing as plugged in to long the block heaters have their own temp control so after a certain point it should shut off (~200deg F). Most people around here run them on a timer which so long as you never leave way early works great if you forget to change it for daylight savings or for some reasson have to leave an hour or 2 early you might be in trouble though (takes ~3hours for full heat on our trucks so most set the timer for 2-3 hours)