I have been doing some small digging, and looking at major Bethesda Studio releases dates and development time that have taken place since Morrowind. Quick Disclaimer: I am not including FONV for the sake of that it was developed by Obsidian and not Bethesda. Without further ado, here we go.

The Elder Scrolls III: Morrowind Release Date and Time: May 1, 2002. 4-7 Years in Development

The Elder Scrolls IV: Oblivion Release Date and Time: March 20, 2006. 4 Years in Development

Fallout 3 Release Date and Time: October 28, 2008. 6 Years in Development

The Elder Scrolls V: Skyrim Release Date and Time: November 11, 2011. 4 Years in Development

Fallout 4 Release Date and Time: November 10, 2015. 7 Years in Development (In and out of being the main priority, they had to make Skyrim)

Fallout 76 Release Date and Time: November 14, 2018. 3-4 Years in Development

If there are any mistakes please feel free to correct.

If we look at how many years are in between each major game release, then there would be 4 between TESIII-TESIV there are 2 between TESIV-FO3, 3 between FO3-TESV, 4 Between TESV-FO4, and 3 between FO4-F076. This makes the average wait time for a release around 3 years. The average development time would be around 5 years. Now, in a recent article on comicbook.com, author Robert Workman states that in an interview with Todd Howard, Fallout 76 and Starfield have been in planning for 5 years, and that Starfield began development right after Fallout 4 was released. This means that this game has been in development for 3 years, and has been stated by Howard to have been thought about for a decade. That means if Bethesda went the normal route and released Starfield 3 years after Fallout 76, then it would have a 2021 release date and would have been in development for 6 years, which is close to how long Fallout 4 was in and out of development. Howard has also said in an interview that Starfield and TESVI may be released on the next console generation, and with both Sony and Microsoft expected to release their new consoles in 2020, 2021 doesn't seem too outlandish. That could mean that TESVI could be released 3 years after Starfield, which would make sense since Bethesda has now announced the two games they said would be before TESVI. This means that TESVI could possibly be released in 2024. Howard has stated that just this year have started and are in very very early development of the game, which would mean that by 2024, TESVI would, like Starfield, have at least 6 years in "in and out of focused" development. Again, if there are any huge mistakes let me know, I'd love to hear your theories, hopes, arguments, or anything about the games. Remember also, I do not think my theory is fact, until Bethesda officially announces release dates, then stay on your toes. They are know to be some wild and unexpected peeps.