To be specific any time they feel you may no longer meet the requirements of your certificate (which is pretty vague, they can do it on whatever whim). Certain incidents, whether pilot-induced or not, are guaranteed 709 rides: gear up landings, fuel exhaustion resulting in a crash...
I've never heard anybody who got a 709 ride ever complain too much about it. They're usually pretty brief and to the point reviews of the specific issue at hand.
The number comes from the us code section that permits the FAA to do this: 49 USC 44709. It used to be a 609 ride years ago before the sections got renumbered.
True, it is non specific about when they may retest you, "at any time", however, in reality, to get a 44709 ride, you have to **** up. The FAA isn't looking for busywork. Note, it is also not specific that the retest must be in a plane and is often satisfied with an oral interview depending on the event.