From your question, it sounds like Talc was an arbitrary choice whose main purpose was to emphasize the dwarves’ ability to make the rocks soft. So, in this answer, I’ll just assume that breaking the rocks requires a negligible amount of energy, and that the hard part is actually removing them from the mountain.
I’ll also assume that you have lots of dwarves, so that manpower (dwarfpower?) doesn’t become a limiting factor. I’m also neglecting whatever time it takes for dwarves to build safety infrastructure (e.g. wooden supports, pulleys, etc.) because, ideally, they can work on rigging that stuff up in parallel with other dwarves as they mine, leaving the net time unaffected.
Here are some parameters involved in our calculations:
* $r$, the radius of a tunnel through which a single rock-carrying dwarf/donkey cart can travel. Given dwarves’ typically small height, $R\approx 1\space\text{m}$ should be a good estimate, but you might be able to get it even smaller.
* $k$, the number of rock-carrying dwarves that can fit in a unit length of tunnel. I’d guess every dwarf needs about $2\space\text{m}$, or maybe $4$ to $5\space\text{m}$ if they have a mini-donkey-cart as well. So let’s say $k\approx 0.25\space\text{m}^{-1}$.
* $d$, the horizontal depth (towards the core of the mountain) at which your mountain hall is situated. This is also the approximate distance of a tunnel connecting the mountain’s surface to the underground mountain hall. This really depends on how deep you want to go - perhaps $D\approx 50\space\text{m}$ is a reasonable distance.
* $V_H$, the volume of your finished mountain hall. 441
A large FIFA soccer field is about $120\space\text{m}\times 90\space\text{m}$, so let’s say your mountain hall is about $120\times 90\times 90$ or $9.72\cdot 10^5\space\text{m}^3$. If you had different dimensions in mind, just plug your volume into the formula at the end of the answer.
* $V_D$, the volume of rock that a single dwarf (together with a donkey-cart, perhaps) can carry in one trip. They’re tough little buggers, so I’ll estimate perhaps $0.5\space\text{m}^3$.
* $s$, the speed with which rock-shuttling dwarves-with-donkey-carts travel. Given that they have short, stubby legs, are carrying rock, and are moving over bumpy terrain, I’d estimate around $0.3\space\text{m}/\text{s}$.
Now, before we start calculating, we have a choice: are we willing to dig lots of tunnels in the mountain in order to speed up excavation? This might be disadvantageous later on, because there will be lots of different entry-points through which enemies might sneak into your hall. However, you should be able to fill up the tunnels afterwards, or guard them with dwarves, or maybe you actually like the prospect of having many entrances/exits. We’ll discuss this again at the end.
A good strategy would be to build these tunnels in adjoined pairs - one for entering the hall with an empty cart, and one for dwarves to shuttle rock away (like two lanes on a street). Entry and exit can’t take place in the same tunnel, because entering and exiting dwarves are traveling in opposite directions.
Let $n$ be the number of such tunnel pairs. The volume of rock that we must remove in order to excavate $n$ tunnel-pairs with radii of $r$ and depths of $d$ is equal to $8\pi ndr^2$. However, since you have unlimited manpower, you can excavate all of these tunnels simultaneously. The rate at which dwarves remove rock from the tunnel will be equal to
$$V_Dsk$$
Which means the amount of time it takes (in seconds) to finish the tunnels is equal to
$$\frac{4\pi dr^2}{V_D sk}=\color{green}{\frac{4\pi dr^2}{V_D sk}}$$
Hang on to that expression - we’ll come back to it later.
Now we get to the fun part - actually carving out the rock for the mountain hall. 771