Five-Year Plan

After an American TV series has completed five seasons, usually about 100 episodes, it can be sold for syndication. This makes the five-year point an important milestone for TV series. Which means it often becomes a milestone for the characters too, with some crucial event scheduled for five years in the future. The TV business being what it is, the shows are often cancelled before the milestone is reached. Of course, five years is a nice round number so not all mentions of it may be related to syndication. Even the title comes from the Soviet Union's economic Five-Year Plans, which presumably didn't have anything to do with TV.

Contrast The Chris Carter Effect, in which a series gradually begins to appear to be made up as it goes along.

Live Action TV

 * The most obvious of examples is Babylon 5, which was planned in a five-year arc for exactly this reason.
 * J. Michael Straczynski's followup Crusade definitely had a five-year plan, with Gary Cole and friends searching the universe for the cure to a plague that was put in the atmosphere of the Earth but which wouldn't have adapted until five years were gone (when it would kill everyone on the planet). They didn't even get halfway through the first season.
 * Word of God says that the plague would have been cured in the second or third season and been replaced by a conspiracy arc.
 * In the remake of Bionic Woman, Jaime's body will reject the upgrades in five years, with lethal consequences.
 * In Dollhouse, the Actives are meant to be released with their original personalities after five years.
 * In a subtle example, Smallville has Clark Kent saying it'll take him about five years to adjust to being an alien. It could be coincidence -- except the creators said that "100 episodes" would sound funny.
 * In Cupid, the title character must unite 100 couples before being released from Earth.
 * Averted in The Sarah Connor Chronicles: Judgement Day is only four years in the future when it could easily have been five.
 * Star Trek: The Original Series had its original "five-year mission", announced at the start of every show. Of course, it never got that far.
 * Unless you count Star Trek: The Animated Series, of course, which features the same USS Enterprise as the original series, the same crew (bar one) as the original series, and had two seasons... which would, technically, bump it up to a complete five year mission.
 * Brimstone had 113 damned souls for Zeke to hunt down. It was canceled after a half-season, so things didn't really work out as planned.
 * In Disney Presents The 100 Lives of Black Jack Savage Black Jack (a ghost) and his live partner have to save 100 souls in order to compensate for the evils they have done. Since only 7 episodes got aired, I guess they're both roasting in Hell right about now.
 * Unusually for an episodic show, every episode ended with a tally showing the viewer how many lives the duo had left to save. Some episodes had them saving multiple lives, in others they didn't get any. It probably would have taken them about 5 years worth of episodes to get to 100... if the show hadn't been canned.
 * There's Supernatural, which was planned as a five-season arc from the very beginning; watching those five seasons makes this pretty clear. Why there's a sixth season in the cards, well...
 * In fact series creator Eric Kripke has admitted that he didn't have a five year plan when the pilot was picked up. The five year plan was developed during season 2 and even then he admits that somethings (such as the Croatoan virus) fit so well it gave the illusion that they'd been planned all along when they had not.
 * Odyssey 5 started with the crew of a space shuttle seeing the Earth getting destroyed, and then getting swept back in time five years to try and stop it.
 * The title character in 100 Deeds For Eddie McDowd was turned into a dog who sounded first like Oz and then like Wayne Arnold, and he had to do 100 good deeds before turning back into a boy.
 * Unlike many such series, he did make some progress -- by the time it was cancelled he only had.
 * Buffy the Vampire Slayer died (again) at the end of her 5th season (in a potential series finale, before the show was picked up again after the WB and UPN merged to become the CW).
 * Word of God is they originally decided to blow up Sunnydale, but then realized it would be too expensive to do at that point. They used that one later on for the final finale of the series.
 * The Merlin writers have often stated that they have a five year plan, but that there's room for expansion if the show continues to be popular.

Western Animation

 * South Park had an episode that revolved around pro- and anti-war activists coming to violent protests, while the boys attempted to write a report about what the Founding Fathers would have thought about the Iraq War (simultaneously being pulled by people on either side)... that is resolved in a giant musical number, ending in for the war, against the war, WHO CARES? ONE HUNDRED EPISOOOODES!
 * If you take the original three seasons from Star Trek: The Original Series and add the two (technically) seasons of Star Trek: The Animated Series to their quota, you end up with the complete "five year mission" talked about in the opening credits.