Seems like 4 months is minimum at this moment (I would guess it's more like 6 is "minimum", if the update is already in decent shape, like 4m to finish the changes already done on the other tracks (if there was nothing done yet, then is 1 month enough to revamp something like SC and FB? IMHO not, so I hope they are doing these reports sort of from "buffer" and then 2m to package it wholly, test, fix last minor issues, etc). If those tracks will need more time, and there will be reasonable chance to finish tire physics, etc... then including the unforeseen issues in test patch period and Scawen's perfectionism I guess it's more like 12m ahead... so Christmas may be possible, if they will commit to it and keep the change list sort of fixed during that year period (and there will be no more interruption, like that DDoS thing which set this off by 3-4 months I think?).
Disclaimer: these are completely made-up numbers by my own guess, I don't have any extra info, only going by info in forums and my own experience from game dev era...
5tag: to release them per-track would require the LFS.exe to contain both old and new 3D engine at the same time.. probably not worth the fuss (I believe there would be lot of fuss and mess from programming side, also editor tools would have to stay in two-variant for longer which further PITA), especially as LFS players should be now well accustomed to the pace of LFS development and how the patches trickle to them. So it makes lot of sense to rather switch to new 3D engine in one go, even if that takes quite some time.