Unless I totally misunderstand, they can listen in immediately; the 72 hours is how long they have to ram the retroactive paperwork through.
You totally misunderstand. The 72 hours is a useless and unused option, because it takes weeks or months to properly prepare a FISA warrant. Before the FISA court ever sees a request for a warrant, it has to pass internal muster, a process designed to weed out anything that wouldn't be a slam dunk approval. The FISA court is not a rubber stamp; it's a tough standard, and the judges are very strict. There's no way to 'ram' anything through them, emergency or not.