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This past week, Deltarune: Chapter 2 was released and contrary to initial statements, it was released for free to everyone. The wait between Chapter 1's release and Chapter 2's release was quite long. A period of about three years spanned between the release of the first two chapters.
Thankfully, the waits between Chapters 3, 4, and 5 won't be nearly as long. In fact, creator Toby Fox says that he plans to release all three of the last chapters at the same time. Fox says that "it's hard for both creators and fans to go a long time without a release."
Unlike Chapters 1 and 2, Fox does say that the next three chapters will come with a price. Of course, he also said that about Chapter 2 but it ended up being free. While a price isn't known yet, Fox does say that it will be more than Undertale, which currently goes for $14.99 (USD) from most retailers.
Right now, there is no word on when those next three chapters will be released. It could be months or it could be years again as the wait was for Chapter 2. Until then, the first two chapters can be played for free from Steam and Itch.io.
Instead, he is now planning to release 3 chapters before working on the remaining 2, as a faster release cycle. Those 3 chapters will be bundled with 1 and 2, and sold as retail (likely digital only). No announcements as to how 6 and 7 are released, but I would guess they would be DLC.
Otherwise final chapters released at the same time would essentially be Toby's original plan, which doesn't shorten the release cycle.
To be fair, you have to play Chapter 2 or watch someone play it to know this, as I couldn't find any info on the game potentially having 7 chapters outside of the chapter selection menu.