Transformation At the Speed of Plot

All pathogens progress at a rate of maximum dramatic tension.

This usually means that any effect that has already been established to occur or progress at a certain fixed rate or timeframe will suddenly progress much faster if it reoccurs later. In more Egregious cases, Headscratchers can ensue -- why is the latest vampire victim-of-the-week crawling out of her grave not twelve hours after their burial, when vampires are supposed to wait three days before rising from the dead?

Despite the title, this is not limited to transformation; it can also apply to illnesses or disease (and is probably one reason why Soap Opera Disease lasts forever).

Take the infection rate of a vampire or zombie bite for example: The Red Shirt who was killed in the opening scene will undoubtably be a full zombie when he is discovered five minutes later, but when The Hero's main Love Interest is bitten and infected, she will suffer a painful and slow progression over the course of several hours (or even days!), successfully hiding her infection from the others or fighting it off until the very last moment (usually long enough to say her final goodbyes). But during their Last Stand against the ensuing Zombie Apocalypse, the former love interest bites the Hero's best friend, and like the Red Shirt who was the first victim, he becomes a full zombie within minutes, if not immediately. (And if some Big Damn Heroes don't show up to save the day, Only the Author Can Save Them Now....)

This can be sometimes justified based on the location or severity of the affliction -- if a character suffered repeated zombie bites to vital locations (such as near the neck or heart), it's logical that they will progress far more quickly than someone who was merely bitten once on the hand and let go. After all, in Real Life, the progression rate of an illness varies with the nature of the individual pathogen, its concentration relative to the victim's body mass, and how well it can circulate through the victim's bloodstream. But in fiction, regardless of how it is established during exposition, the formula ultimately boils down to the victim's importance in the plot, and whether they were affected near the beginning, middle, or end of the work itself.

Distinct from instances of a character taking so long to change into battle mode (often in Stock Footage) that the villain could've just shot him; that's Transformation Is a Free Action.

See also Traveling At the Speed of Plot (for geographic distances), Magic Countdown, and No Ontological Inertia.

Comic Books

 * Post Crisis, Superman's origin was changed so that his powers developed slowly as he grew up, storing solar radiation in his body. However as an adult, staying away from the sun for a just a few days quickly drained his powers. Even worse, other kryptonian characters ignored this and charged up rather quickly.

Film

 * Despicable Me: Surprisingly justified.
 * Wikus in District 9. He stays just human enough just long enough for the movie to play out (and for the audience to identify), while transforming quickly enough at first for him to use alien technology.
 * Fantastic Voyage has a 1-hour time limit before the surgical team will begin to regain their original size.
 * In the novelization by Isaac Asimov, the degree to which you are shrunk affects your time at that size. It is stated that being shrunk to half-size would take years to wear off, but the extreme size reduction needed by the team means they only have a single hour.
 * The gestation period of xenomorphs in the Alien series. The first film has several hours between the victim getting attacked by the facehugger to the chestburster leaping out, and another several-hour interval before the xenomorph matures. In Aliens vs. Predator, a group of people is implanted and the temple is swarming with mature xenomorphs in a matter of minutes.
 * In the second Blade movie, a new strain of vampire virus creates Reapers. The transformation from a vampire to a Reaper is shown early in the movie to take place quickly with considerable pain and Body Horror. However,  gets plenty of time to watch Blade's fight and then the sunrise.

Literature

 * The Drow elves in Forgotten Realms. It was established in Drzz't's origin trilogy that it took him months to adjust to suface daylight; later drow stories would have them up and about in the sun sometimes almost immediately.

Live Action TV

 * Justified in a Get Smart episode where Smart has been poisoned with a toxin that kills in 24 hours. CONTROL's attempts to reverse-engineer it result in a version that kills in sixty seconds, which he uses against the doctor who developed the original poison to acquire the antidote.
 * Justified in Doctor Who, "The Waters Of Mars". The zombie-like infection is actually a microscopic Puppeteer Parasite, and can decide for itself when is the best time to take over its victim.
 * Not justified in "Amy's Choice," where the attack that instantly dusted everyone else we saw it hit gave Rory enough time for a tearful goodbye before gradually dissolving. Of course, it was All Just a Dream, so who says it has to make any more sense than a "cold star?"
 * If you watch, Rory is hit with what looks to be a smaller and more indirect dose of the gas, hence he dusted slower.
 * In the pilot of Stargate: SG 1 the team finds a long list of stargate addresses, but since the stargates have drifted since the list was put down, they have to be re-calculated by their computer. Carter states that with this process they can discover two or three new stargates a month.
 * Of course in a later episode they find a complete and up-to-date map of stargate addresses and drop that conceit.

Western Animation

 * Spongebob SquarePants: In the episode "Best Frenemies," the Kelpshakes that everyone is drinking give them green shag that covers their entire bodies, but not until the end of the episode. However, Krabs and Plankton, who have been avoiding drinking them for the entire episode, finally drink it before Karen can warn them of the toxic ingredient. The Kelpshake effect happens almost immediately after they discover that everyone else has it.
 * In one episode of Disney's Aladdin, Jasmine was given a mysterious "beauty potion" that actually began slowly transforming her into a naga. Cue a long trek to locate the means to reverse it. Ultimately, Aladdin declares he would rather stay with Jasmine than leave her behind, and uses the potion on himself -- with immediate results.
 * In one episode of Ben 10, Ben is scratched by a creature who has been identified as the Yenaldooshi by a Character Of The Week, and spends half of the episode slowly becoming more and more like it. In the end, it's revealed that it was really nothing more than an alien wolf-like creature, and the transformation was due to a quirk of the Omnitrix that allows it to learn and mimic the form of any alien that touches it. Every other time this ability comes into play in later episodes, the form is added automatically and the transformation is as quick as Ben's usual shape-changing.
 * Handwaved by saying something like it was stuck inbetween collection mode and something else, so it caused a gradual change.

Video Games

 * The protagonists of Final Fantasy XIII are made into l'Cie by the fal'Cie Anima: their Focus mandates that they must destroy Cocoon, or else transform into mindless, shambling Cie'th. Each of them bears a brand that acts as a timer, indicating how long they have left: however, the importance of this time limit is inversely proportional to how determined the protagonists are to Screw Destiny.
 * In the Adventure Mode of Super Smash Bros. Brawl, the first cutscene depicts Mario and Kirby transforming from trophies to their animate forms toe-to-head. After that, any time a character is revived from his or her trophy state, the transformation is a simple white flash on the screen. If the trophy in question was lying on its side when it happens (which is most often), the character is then shown either getting up or already standing.