Yeah, the controller won't re-start if it thinks it's hot. Two minutes is pretty much always safe; I seem to recall someone who said he could "always" go for three minutes. But why push your luck? Preheat empty or preheat full, but why both? I've done both, and prefer the beans-in mode, which may have something to do with a house that does get kind of cool at this end of the year.
Sweet Maria's, Behmor, Encore, Bonmac, CCD, Pavina - fun AND good coffee