That's straight from Bethesda's Todd Howard, who shared a rough road map of the studio's production pipeline in a new interview with IGN.
"Yes, Elder Scrolls 6 is in pre-production and, you know, we’re going to be doing Fallout 5 after that, so our slate’s pretty full going forward for a while," he said. "We have some other projects that we look at from time to time as well."
Last year, Howard confirmed that the studio had created a "one-pager" for the next Fallout game, but there was no indication that the project was anything more than a basic premise and setting.
With The Elder Scrolls 6 presumably a ways out from full production amidst ongoing work on Starfield, which was delayed to 2023 fairly recently, we can reasonably assume that Fallout 5 won't be a playable game for anywhere from eight to upwards of 10 years.
Of course, that's assuming the studio doesn't see any massive internal changes or expansions. For reference, Fallout 76 – which a new report describes as a "nightmare" project that drained staff and even pulled devs off of Starfield and Redfall – came out in late 2018, and Starfield was also in the works when Fallout 76 was in active development. Bethesda does not release games quickly, and it doesn't look like that will ever change.
"They do take a while," Howard told IGN, "I wish they came out faster, I really do, we’re trying as hard as we can, but we want them to be as best as they can be for everybody."
Howard recently said that Bethesda is currently putting the "finishing touches" on Starfield following its delay.