Percy fully learns the exact wording of the Great Prophecy at the end of The Titan’s Curse , when Chiron finally recites it to him after the quest is over and the gods have discussed his fate.

Quick answer

  • Percy spends most of the original series knowing only that there is a mysterious prophecy about a child of the Big Three who will make a world-shaking choice at sixteen.
  • The full Great Prophecy (the one about a half-blood of the eldest gods reaching sixteen and either saving or destroying Olympus) is revealed to him explicitly at the end of The Titan’s Curse.
  • Later, in The Last Olympian , he finally understands how every line of that prophecy applies to him and the events of the war with Kronos.

In the books vs. the show

  • In the Disney+ adaptation, Annabeth explains the Great Prophecy to Percy early in the Sea of Monsters arc (Season 2, Episode 3), while they are on Luke’s ship, the Princess Andromeda.
  • In the novels, the reveal is paced more slowly: Percy hears hints, guesses parts of it, and only gets the full text directly from Chiron much later, after several quests and close calls.

Why everyone keeps it from him

  • The gods and Chiron are afraid that if a hero hears a prophecy too soon, they will try to change it and accidentally bring it about, which is a running theme in Greek mythology and in the series itself.
  • Annabeth in particular hesitates to tell Percy in both book and show versions because she worries he will resent her, and because “knowledge isn’t always good for you” when it comes to prophecies.

TL;DR: Percy finds out about the Great Prophecy in pieces, but the key moment when he finally hears the full prophecy text is at the end of The Titan’s Curse in the book series, with a somewhat earlier and more direct reveal in the TV adaptation’s second season.

Information gathered from public forums or data available on the internet and portrayed here.