https://www.gamespot.com/articles/more-fallout-76-beta-details-coming-soon/1100-6462015/

The beta for Bethesda's much-anticipated Fallout 76 begins in October, but we still don't know all that much about when in October it'll launch, what it will contain, and how long it will run for. It sounds like more details are coming about the beta soon.

Responding to a fan's question about the Fallout 76 beta length, Bethesda marketing boss Pete Hines said on Twitter, "We'll have more information for you on that soon. Hopefully this week."