One of the Wonders earns the player, in its second stage, the right to go through the discard pile of previously played ages, choose one card and build it for free. Is this a one time deal, to be done at the conclusion of the age during which the player completes the stage, or can the player now "dumpster dive" in the discard pile at the end of every subsequent age as well?
You can use this ability one time only: exactly at the end of the turn in which this stage is built (not at the end of the Age!).
(And note that it’s not only for cards from the "previously played ages". The discard pile may also contain cards from the current Age and even the current turn.)
The relevant explanation is on page 8 of base game rules, "The Mausoleum of Halicarnassus":
Clarification: this special ability takes place at the end of the turn in which this stage is built.