Historical Rules · 1 October 2010
Rule 611.2b
Official source text from the 1 October 2010 Rules release
Some continuous effects generated by the resolution of a spell or ability have durations worded "for as long as . . . ." If the "for as long as" duration never starts, or it ends before the moment the effect would first be applied, the effect does nothing. It doesn't start and immediately stop again, and it doesn't last forever. Example: Endoskeleton is an artifact with an activated ability that reads "{2}, {T}: Target creature gets +0/+3 for as long as Endoskeleton remains tapped." If you activate this ability and then Endoskeleton becomes untapped before the ability resolves, it does nothing, because its duration--remaining tapped--was over before the effect began.