Nor in television, for that matter. Indeed, Babylon 5 could be regarded as the longest miniseries in history, because from the initial concept, JMS intended it to run for a maximum (and preferred) length of 5 years, beginning the "Vorlons vs. Shadows" arc in the first season, and slowly moving from mostly standalone episodes to mostly arc episodes until the arc was resolved.
It's a bit disingenuous to use "mini-" for a 5-year, 110-episode series, though. Even "limited series" would be pushing it, since 5 years is a long run for a TV series; most shows are lucky to make it to a second season, let alone a fifth. (Before
Star Trek: TNG, the only American SF/fantasy shows that made it longer than 5 seasons were
Adventures of Superman and
Bewitched.) Indeed, many shows today are designed with the expectation of running 5 years; it's become pretty much the standard target length for first-run syndicated or cable shows.
The term "miniseries" was originally used in TV to apply to productions like
Rich Man, Poor Man and
Roots that ran for 8-12 hours and were aired over a limited period (7 weeks for the former and 8 consecutive days for the latter). Later on, in the '80s and '90s, it became standard to use it (somewhat inaccurately) for 4- to 6-hour productions that were aired in movie-length installments over 2-3 consecutive nights, like the two
V miniseries or the various Robert Halmi-produced miniseries that the SciFi Channel used to air. Generally
the term has been limited to productions that have a single finite run that's shorter than the 13-episode minimum season length of a typical ongoing show. (Although there have been miniseries that have spun off ongoing series, including
V and SciFi's
Battlestar Galactica.)
(Particularly telling is that the final episode was written, filmed, assembled, and ready for airing by the end of the fourth season, and then, when the fifth season was given the green light [not by the original network, as I recall], held for a full year.)
That's because the show was cancelled in syndication after season 4. Once JMS learned that the syndication package (the Prime Time Entertainment Network, PTEN) wasn't going to pick up a fifth season, he reworked his plans to compress the last two seasons' arc into just one season, which meant moving up "Sleeping in Light" to be the 4th-season finale. But then TNT picked up season 5 and saved the show from cancellation, so JMS wrote a new finale for season 4 and postponed the already-filmed "Sleeping in Light" until the end of season 5 where it had been originally meant to go. He had to piece together a new season 5 arc from a mix of leftover subplots excised from season 4 and moved-up plans for post-B5 movies and series, which is why season 5 feels like something of an afterthought separate from the arc of seasons 2-4.
So far from being the result of meticulous advance planning, the early filming of "Sleeping in Light" was the result of things
not going to plan and having to be changed on the fly. The myth of B5 is that every last detail was perfectly planned out 5 years in advance, but in fact, JMS's plan for the series went through constant reworkings due to external circumstances -- the syndicators demanding cast changes after the pilot, other cast changes forced on the show for other reasons, story editor Larry DiTillio leaving after season 2, and so forth. What made his plan work is that it
wasn't ironclad, but highly flexible, changing constantly to adapt to the unexpected. JMS was fond of quoting the military saying "No plan ever survives its first encounter with the enemy." So he understood the need to be adaptable. "Sleeping in Light" is an example of that. He moved it up to the end of season 4 when he learned PTEN wouldn't pick up season 5, but he did it in a way (by time-jumping it decades into the future) that made it easy to postpone it another year if he managed to get a season-5 pickup some other way -- or possibly even longer if the syndicator or network had insisted on a season 6.
