The Force Awakens. Previously, there would be some kind of time jump between episodes ranging from a single year (The Empire Strikes Back to Return of the Jedi) to an entire decade (The Phantom Menace to Attack of the Clones). Given the way The Last Jedi ended - particularly with the Resistance in dire need of building up their forces - many assumed there would need to be some kind of sizable gap before Episode IX picked up.
Indeed, there was some evidence suggesting a time jump was in order, most notably comments from John Boyega about Finn's new look. However, this was always just speculated on the part of viewers, as Episode IX's placement in the timeline was a detail kept under wraps by Lucasfilm. Now, however, there's new information available on that front.
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According to Empire (hat tip DiscussingFilm), Episode IX is set one year after The Last Jedi. This means the film will formally be 35 ABY (after the Battle of Yavin), since both The Force Awakens and The Last Jedi were set 34 ABY.
Some might have been expecting a larger jump forward, but one year should work fine for the film's purposes. That seems like plenty of time for both warring factions to regroup as they become embroiled in an all-out conflict for the fate of the galaxy (which will likely be well underway by the time the opening crawl fades away). Additionally, it gives returning characters like Rey, Kylo Ren, and Finn ample opportunity to have already adjusted to their new situations and responsibilities, allowing them to face new challenges in the sequel trilogy finale. Ideally, fans will see real growth in the principal players, illustrating how far they've come since their journeys began - similar to Luke Skywalker in Return of the Jedi.
Another benefit to Lucasfilm implementing a time jump for Episode IX is that now all the other areas of canon have unexplored territory that will be ripe for novels, comics, and other materials. While there was some content set between Force Awakens and Last Jedi, the story group was obviously limited with what they could do because of how close the movies were. Some of the best Star Wars narratives in the Disney era are publications that canonically take place between films, so it'll be interesting to see what Lucasfilm comes up with as they prepare to bid this era of the franchise farewell.
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Source: Empire (via DiscussingFilm)