The Fallout 76 beta doesn’t have concrete date yet, but Bethesda has answered a few key questions over on its official beta FAQ page.
The FAQ page has recently been updated with a new question which asks whether the beta will cover the whole game, and whether progress made in the beta will carry over to the full release in November. The answer to both of those questions is YES.
Bethesda says that its current plan for the beta is for it to include the full game, and for all of your progress to then carry over to the finished release.