Out of curiosity, I asked on Area 51 about the timeframe, and I received a response from someone who works for Stack Exchange.
The good news is that they're working directly with Salesforce on the branding of this site. However, it may take a while, yet.
We don't have a set duration for betas; sites "graduate" (I think what you mean by launching - getting a custom site theme and whatnot) whenever the our community managers' periodic evaluations deem them ready and our designers have time to create the design. We are actually currently working directly with Salesforce on an initial design to propose to the community. Because we're working with SF, and we're also going to be rolling out a new network-wide top bar design that will impact future site designs, this process is moving slower than normal, but we're moving towards it.
I'm not going to venture a guess at a timeframe because I'm not closely involved in this project, but keep an eye out on Salesforce.SE's meta site for announcements from community managers or designers in the coming weeks and months. Until then, there is nothing functionally incomplete about our beta sites; the biggest change that comes with graduation is cosmetic (the custom theme). – Laura
So, they're still working on it; we just need to keep checking back here on the progress in the next few weeks/months.