"2. My ventilation ducts will be too small to crawl through."
— The Evil Overlord List
Air ducts in a modern office are 18 inches wide. So if you need to make a quick escape and are older than 4... you won't fit. A more viable option is the subceiling. Once you're in the subceiling, you can get to a bathroom or closet, and make your way out from there.
—Michael Westen, Burn Notice
"There's Nazis in the walls!"
— Danger 5
I love 1958, no-one's seen Die Hard. Or Alien. Or Die Hard 2, or Aliens. [...] There's always a ventilation shaft!
"Sounds like Thor, god of thunder, is trying to sneak into my building!"
— The Mythbusters busting this trope (or, at least, doing it without being noticed...)
In the movies, ventilation shafts are the escape route of choice for heroes trapped in industrial buildings. They're clean and roomy and soundproof, and will take you anywhere you want to go all, like a Habitrail system for the beleaguered protagonist on the run. I don't know where Hollywood buys their ventilation shafts, but they don't use the same supplier as the museum.
— Hope Adams, "Chaotic"
"Attention test prisoners attempting to escape though the air ducts: I don't know what nonsense you learned on TV, but in real life, air ducts just lead to the air conditioning unit. It's also pretty dusty, so if you've got asthma, chances are you're gonna die up there and we'll be smelling it for weeks because — again — the air ducts aren't a secret escape hatch. They're how we ventilate the facility."
—Warden Cave Johnson, Perpetual Testing Initiative
Oh yeah, no one would think of looking for us in THE VENT!
— Cale Tucker, Titan A.E.
Hermes: I thought we installed alien-proof air vents!
Hondo Ohnaka: Well?
Gwarm: We've secured the ship, but she seems deserted.
Hondo: Imbecile! That is the oldest trick in the book. The runts are probably hiding in the VENTILATION SYSTEM.
Gwarm: Oh, right. Didn't think of that.
Hondo: Why does that not surprise me?
— Star Wars: The Clone Wars, "A Test of Strength"