How long would it take to build an underground city?

Question

In my fantasy world, there is a race of ghouls. They are a knit-tit community, and while they built underground cities, including reforming abandoned mines as living places, they eventually kept living down there, especially since they became full-on cities and not just settlements. My question is how long would it take to build an underground city? Here is some of the description:

1. The population in these cities can range in the 1,000's
2. These cities have sewer pipes, ventilation, and have well-systems to extract water
3. There are not just homes and apartments, but also markets, prisons, workshops, churches, brothels, and just all the stuff you would see in a city during the Medieval Period and Renaissance.
4. Some cities were not originally cities. Some are re-built mines, and usually the refurbished structures that became cities and not towns were catacombs or large dungeons.
5. In re-built cities, there is the problem of fighting off feral creatures underground.
6. There are large roads to transport carriages, small commercial boats, and goods, and these become more common as they start to trade more.
7. Like any underground structure, these cities are loaded with giant pillars to keep it from collapsing, although many cities used the advent of magic
8. The magic used is energy-based, it transforms energy into different forms, and has to follow the rules of thermodynamics.

For knowing what it looks like, these are some pictures found that resemble some of them, or at least the style:

Answer 1

The speed of excavation very heavily depends on the tools available, the machinery available to remove the ground and of course the material the ground is made from.

Just a few examples:

• it took less than a year to dig 15 kilometers under the alps with pneumatic drills, a construction rail, and dynamite.
• However, it took 17(!) years to drill a pair of 57 kilometer long tunnels and construct the rail, just because that tunnel is also much deeper.

Both these tunnels are made through somewhat similar stone, but the complications and dimensions of the more modern tunnel made it take much longer per kilometer.

Answer 2

About as fast as a normal city

The time it takes to build an underground city is not significantly different than an above ground city. There are many YouTube videos out there of 1-2 people making underground homes in 30-150 days using only primitive tools; so, building the city is quite easy, the hard part with underground cities is economics.

When you build underground it makes lighting much more expensive because you never get daylight. Above ground living also tends to make easier supply lines to resources that you need in bulk from the surface like food and firewood. That said, underground homes are more defensible and thermally stable.

So to to develop an underground city quickly you need to change the economic factors to favor it. For example, if your ghouls have a combination of extraordinary night vision, a preference for raw foods, sensitivity to the cold, and/or a heightened need for secrecy because they are a hunted race, then below ground construction could become a more economically desirable choice. As long as you can create enough demand to live underground, the city will grow just as fast as any other.