My scientific wild-ass guess, as of current, is: up to two months for getting the requirements and specs, then about 9 months for a first (beta) release of the platform. That puts us in position to launch in about a year from now.
The goal I had in mind when joining this project (since I first made myself available for SE members that would be willing to kickstart the required efforts) was to rebuild a platform that works as well as SE (and my experience is that it works damn well), just without belonging to SE but rather to the community. In that light, I expect us to be deviating as little as possible in terms of software. We’ll do better where it can be done, but as I mentioned in What are we trying to build? - #2 by Marc.2377, I (for one) don’t expect the final product to be wildly different from current SE.