418.3d.

Some effects from activated or triggered abilities have durations worded "as long as . . . ." If the "as long as" duration ends between the end of playing the activated ability or putting the triggered ability onto the stack and the moment when 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 as long as Endoskeleton remains tapped." If you play 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.