It's a finger in the air answer as the programme will last until any large bugs and implementation issues are ironed out and then a 'stable' release pushed out for final rollout testing.
It all depends on issues found, feedback, unit testing, and inclusion of any new features along the way that need validation.
I've been part of Betas that last 12 weeks plus & some that are less than 8 from 1st Beta version to a final; with some having 4 version releases during beta and others having 2.
With UI7 Beta being delayed going into public testing due to additional dev cycles and testing, and the fact that (touch wood) from what I've experienced personnaly it being very stable with issues raised on here being mostly cosmetic function tweaks or minor niggles, then I wouldn't be surprised if a 2nd update to Beta is pushed within a couple of weeks and then a 3rd / final official release hit by the end of Jan / early Feb.
It is though one of the unknown variables
