It depends on how one would define "functional".
To create something that runs without crashing and has the meshes in the correct place, would maybe take two-to-four weeks. The biggest problem is that no source code is available, and you therefore have to guess the mesh positions by trial and error. Not a fun experience.
If one were to include animations (solar panels, radars, etc.) probably another two-to-four weeks).
Then, adding thrusters, rocket staging, and other manual functions, maybe four weeks. Add one-to-two more weeks for the HUD info.
Finally, you may want to implement launch autopilots, automatic attitude control, realistic aerodynamic properties, etc. This can take everything from six weeks to half a year, or more.
There have been people spending an entire bachelor thesis on modeling the Soyuz aerodynamics.
The week numbering includes an hour or two of work approximately every day.
For reference, I started working on the Project Mercury upgrade in September 2019. I considered it ready for an initial release in February/March 2020. I had old source code to work from, which the one upgrading the Soyuz will not have.
Also, a lot of the parameters (thruster values, vessel mass, autopilot pitch programs, abort system limitations) came from NASA documents and archives. I highly doubt that the Soviet/Russian Soyuz has the same level of openness. And if it has it, it is probably in Russian.
But this of course all depends on what level of detail you want. It is much easier to make something "good enough", than to make something historically correct.
My advice would be to start with creating something flyable, using VesselBuilder, Multistage2015, GenericVessel, and/or Spacecraft4. This way, you can create something workable in days, instead of weeks, and something nice in weeks instead of months.
Then, you can improve it until you reach the limits of these tools. By now you already have something much better than currently available for Orbiter2016.
So if you or anyone else feels like expanding the addon further with custom modules, it's much easier to carry the fire from there.
---------- Post added at 13:39 ---------- Previous post was at 13:22 ----------
But ah, looking at the Thorton Soyuz manual, it says "You are not allowed to redistribute or use these meshes or textures in other add-ons."
That makes matters worse.
Generally, I believed you would be allowed to release e.g. a Spacecraft4 ini for the Soyuz meshes, as long as the end user got the meshes from installing the original addon (maybe something under fair use?). But I would recommend diving into the
License Mega Thread, and (possibly best of all) reaching out to Thorton him-/herself.
