It’s not a “time” thing. It’s a matter of security. If anyone who took part in an operation is still alive; if anyone who supported an operation is still alive; if anyone could conceivably still benefit from the info; if anyone could conceivably benefit from knowing that we obtained info (and knowing is a benefit in and of itself); any of these can combine to form a damned good reason why you just don’t talk about operations.
You know that crap from WWII that’s still classified? It’s probably still political dynamite; like maybe we assassinated Churchill and replaced him with an American impostor.
Politicians being politicians, will want an occasional bone to toss to the masses. In which case, the various national intelligence agencies dip into their haversack of Dead Ops., change things around to obfuscate times, dates, places, and methods, and say “Here ya go Mr. Pres. Tell 'em all about this one, where we stopped terrorists from pouring deadly bio agents into the L.A. reservoir system.”
When in reality, they stopped some bombmaker in Tunisia from building a bomb for a terrorist cell in Sudan, that was going to hijack an El-Al jetliner and fly it into Mecca. Or something.
Telling the Mayor of L.A. prematurely could’ve spooked the bunnies back underground, where we would have to again locate them, infiltrate them, and probably, again stop them at a later date.
Telling the Mayor of L.A. could’ve spooked some staffer into leaking the information “for humanitarian reasons*,” only to see L.A. dissolve into a bloodbath as everyone scrambles to evacuate, and don’t care who they shoot, maim, or kill to get out of town.
*And that’s ascribing more altruistic motives; it could be as simple as him calling his nice, senior citizen mom at home and telling her to get out of town. NOW! She calls Gladys and Henrietta, her dear, dear, friends, and tells them her Tommy, who works at the MAyor’s Office, overheard a call from the Dept. of Homeland Security, and now he’s telling me to get out of town, quickly.
Now Gladys and Henrietta call two more friends each, as well as each and every one of their grown children. And you see where this goes: L.A. dissolve into a bloodbath as everyone scrambles to evacuate, and don’t care who they shoot, maim, or kill to get out of town.