Keep in mind that this is just my opinion...
The Mushroomian Spacedock never made sense to me in TMP3. If it was the "hub" of spacecraft construction and traffic activity in Earth orbit, why didn't we see it before that? The thing is huge. It would loom like a second moon in Earth's sky, and be plainly visible for many thousands of miles, everywhere in Earth orbit except for the far side of the planet. In fact, it looks so huge that I wouldn't be surprised if you could see it from the Moon.
This is in sharp contrast to the orbital office complex where Kirk first meets Scott before traveling to see the refit Enterprise for the first time in TMP1. The scale of the orbital office complex is much more in line with what we've come to expect of space vehicles and stations in the TOS Universe. When did Fesarius-sized vessels become part of the Earth/Federation repertoire?
It seemed to me to be one of those examples of Star Destroyer / Death Star envy that crept into TREK during those days. It was more concept erosion.
Having said all that, even though the Mushroom sticks out like a sore thumb, I suppose the Federation would have the technology to build one if they really really wanted to. The practicality of maintaining and securing a vessel of that size would seem to be a headache, though.
It would seem to me that if it takes Earth and/or the Federation years to design and build a new class of starship-of-the-line, then a station the size of the Mushroom would take years or even decades. Maybe the station evolved gradually over time, starting as a small office complex during the early days of the UFP, then growing over generations. That's the only way it would make sense to me.
Well, the point of this thread is not if we LIKE Spacedock as a concept or construct... I am trying to ascertain how long it would theoretically take to build it, to establish a rough timeline of when Starfleet might have begun work on it.
See, guys the Spackdock concept makes complete sense.
Even NASA at this primitive time in Space Exploration has planned concepts of an enclosed ship production facility.
http://t2.gstatic.com/images?q=tbn:...8C0gwY&t=1&usg=__xcvuwPne3wuKBYKq_uVYbagzj54=
Designing the Facility
Producing plans of anything is about man power and processing for the job size. If you're designing something this large then it would be important to break the project into section of descending order. Powerplant, Habitat, Dock.
The Design of the Spacedock was already predisposed to a certain amount of alacrity because it's symmetrical which means a large amount of repetition of design per 10 degrees with minor deviation per section. Each Area Level would be given Design Priority Authority which means in the design phase certain sections were more important than others. Any needs or mistakes in utilities or just design in general would conform to the needs of Authoritative section. Sectional Degress would be broken up by utilites, architecture, structural (not in that order)
With a design crew of say, 25,000 to 30,000 of Senior Designer, Drafters, checkers and Nuclear Specialist it's just a 5 to 7 Year design project.
Production
The trick with production is purely about available materials. Often you are always waiting for something and almost everything is on back order.
What we haven't seen in Trek of the 23rd century are orbital production facilities. It doesn't mean they don't exist but without them planetary/STS transit for all those materials is necessary. Amusing that these orbital production facilities were in the 1st of 2nd stages of development for the Federation Civilization (I say 2nd Stage because the Registry shows a fantastic BOOM of ship production from the 2,000's-6,000's in the late 23rd Century to the 20,000 and 40,000's in just a few Decades.) then the production and transit of these materals to location would have been on the scale of 1-10 a 5.
Construction
Every Construction project has a Time Frame and every Time frame is dictated by available funds. The Federation doesn't have a completely capitalist system. So they only dictate is available materials.
With money out the way available materials are abundant but limited by distance, say of around 10 to 15 lightyears. With the combined production of High Council Planets aswell as the Founding Planets Vulcans, Andor, Earth, Alpha Centauri, Tellar (the more advanced civilization carrying the larger load) the Production and Construction load can be adequately distributed.
If WWII is any indication of even primitive production/construction ability on the scale of ships.
In Seven Years Combined AXIS/ALLIED PRODUCTION
(Most Battleships and Cruiser were pre-WWII proucts)
121 Aircraft Carriers
20 Battleships
97 Cruisers
900 Destroyers
1758 Submarines
That tonage alone could be a quarter of the Primary Dock Facility and that's just one Primitive Production and Resource system in 7 Years.
My Conclusion
Mandate and Legislation: 2 Years
Design: 5 Years
Construction: 8-10 Years
Since the Design of the Spacedock is modular it's possible that there was considerable overlap in Design and Production like with the Intentional Space Station. Completed in stages
at 12 or 13 years of Construction.
The project could have began halfway through Enterprises Life Time which would make sense as the dock size fits the Constitution Class Properly.