100 episodes
From Freepedia
100 episodes is considered to be the magic number at which point many television series produced for the United States (which usually run 22–26 episodes per year) are viable for syndication. Although much depends on the length of a show's seasons, this point is usually reached during a prime time series' fifth season.
Syndication
Syndication is often a profitable enterpise, due to the fact that series can run for decades after they stop production. In this way, many shows that do not gain much profit during their first run will still prove to be viable to the network if they can last 100 episodes.
There are many exceptions to the 100 episode rule: shows of fewer episodes that have become syndication successes. The most notable of these is the original Star Trek series which had only 79 episodes available when it ended in 1969, but subsequently spawned ten movies and four spin-off series. What's Happening!! did much better than in its first run on television, despite only having produced 65 episodes. An extreme example is the spy series The Prisoner which has been successfully syndicated for more than 30 years despite having only 17 episodes produced.
On the other hand, a series reaching 100 episodes does not necessarily mean that a) it will be picked up for syndication, or b) that these reruns will be successful.
What this means to fans
Many fans of story arc oriented series—rather than highly episodic ones — consider this a critical time in a show's development. Often, a show that reaches its fifth season or beyond may grow stale or start to lose its audience, especially if it is reported to be picked up for more seasons. This can happen either because a series is changing its tone "excessively" or sometimes not changing enough.
Most long-running series - ranging from The X-Files to Buffy the Vampire Slayer to The Simpsons - will be accused by both critics and fans of "jumping the shark" (that is, of losing the initial qualities that attracted their early fans) once they get to this time. However, it should be noted, for every fan who shares this opinion, there are usually fans who think the opposite. A good example is Seinfeld, which - while many fans think of the early or middle seasons as best - has a large fanbase who consider the final seasons to be superior.
