Here are all the times Dragon Ball Z was meant to come to an end. Akira Toriyama's Dragon Ball franchise was first published as a manga over the course of 11 years, running from 1984 to 1995. During that time, Dragon Ball was adapted into a TV anime series that rocketed the adventures of Goku and his friends to worldwide fame. Even in 2019, Dragon Ball remains the most popular anime franchise at an international level and has spawned countless video games, spinoffs and movies, despite the main story concluding more than two decades prior.
As with any popular media property, there has been a certain amount of resistance to the idea of bringing Dragon Ball to a definitive end, as those with a stake in the franchise seek to keep interest (and income) high. As a result, there have been several points in Dragon Ball history when Toriyama had planned to bring Goku's story to an end, but ultimately carried on producing material.
Toriyama is infamously inconsistent with his interview answers, but has cited several different occasions where Dragon Ball was supposed to end. The earliest point is after the very first arc of the manga, which sees Goku, Bulma and the original gang search for the seven Dragon Balls. Toriyama confirmed this in a 1995 interview, stating that he only expected and planned for the series to last a single year - perhaps more a reserved estimate of Dragon Ball's potential success than a deliberate desire to wrap things up at such an early stage.
Once Dragon Ball started picking up momentum, Toriyama revised his original estimate and, in Vol. 5 of the manga, claimed that he expected the series to only run until Vol. 10. While mangaka are famous for their inaccuracy in predicting how much story is left to tell, this forecast at least confirms that Toriyama originally wanted Dragon Ball to end while Goku was still a child and perhaps even before the introduction of King Piccolo. Only 2 volumes later, however, Toriyama revealed this plan had changed because of Dragon Ball's rising popularity.
Consequently, Goku's adventures continued into the Piccolo Saga, but Toriyama was quoted in the fan questions section of Vol. 12 as saying Dragon Ball would only continue for a "little while longer" after the Namekian's defeat. In hindsight, this sentiment seems laughable, since Dragon Ball's popularity was growing rapidly after Goku hit adulthood and fought against the Saiyans. Still, this revelation strongly implies that the lengthy Frieza Saga was not in Toriyama's plans when he started work on the Z part of the story.
Many fans cite the intended end of Dragon Ball as the conclusion of the Frieza saga, with Goku fulfilling the Super Saiyan prophecy and finally defeating the villain responsible for destroying his home planet. Of course, this moment ended up being the beginning of a whole new chapter for Dragon Ball, rather than an ending, but there are perhaps a few possible signs that the story was supposed to finish with Frieza's death. The most obvious is when Goku is seemingly shown dying on the exploding Namek, and then revealed to have escaped in a space pod, despite clearly going down with the planet. Toriyama himself has never openly claimed that he intended Dragon Ball to end with the Frieza Saga, but many figures close to the franchise have, and Toriyama's former editor, Kazuhiko Torishima, is quoted (via Kotaku) as saying he feels the series should've stopped at this point.
Likewise, the Cell Saga is often claimed to be Toriyama's "true" desired end-point, even though the man himself has never stated this publicly. In fact, Toriyama has contradicted this theory by revealing how he planned to make Gohan the lead protagonist after the Cell Games arc, proving he did have plans to continue the story, albeit not with Goku as the main hero.
Toriyama eventually succeeded in bringing Dragon Ball to a close after the defeat of Majinn Buu and the hiatus lasted a long time - 13 years to be exact. True to form, however, Toriyama revisited his most famous creation with Yo! Son Goku and his Friends Return in 2008, which paved the way for the Battle of Gods movie and a whole new era of Dragon Ball stories on the big screen.
Sources: Kanzenshuu, Kotaku