Background assumptions
To start, using cranes as a metric for roughly the level of sophistication that a steam-punk-era civilization can create, from what I've found, the disassembly process is around the same scale of magnitude as the assembly process. I think this is a good metric for the ratio of assembly to disassembly time on the level as a city as it has some complicated bits of machinery, but is largely just tons of material. Also, the disassembly process of a crane is non-destructive and made for it's pieces to be transported, as, intuitively, the parts of the crane are supposed to be used again elsewhere.
As such, perhaps the answer may just be roughly the sum of the time it takes to construct each building in the city (or at least on the same scale of magnitude).
Time for some estimates
Before I begin, clearly this is a very rough estimate with huge assumptions, but I reckon to be pretty close to an actual value within a scale of magnitude or two.
Mathematically, let's use $\frac{storey}{humans\times time}$ as our base unit of estimation. The goal is to estimate how many storeys ($S$) there are in each building in a city, then estimate how long it takes to build a storey in man-years $(Myr)$.
The Empire State Building was built in about $1.12$ years, took $3000$ workers, and is $103$ storeys tall. Thus, it was made in $\frac{103S}{3000M \times 1.12yr}\approx0.03\frac{S}{Myr}$. Let's take this value and run with it.
Let's assume the average residential home has $2S$, and the average 'skyscraper' has $50S$.
New York City has about $274$ skyscrapers and $761000$ residential homes. 934
In total there are about $1000000$ buildings. Let's use that data to plot a normal distribution for the amount of homes with $n$ storeys. We find that there is a $0.0274\%$ chance of a given building having $50S$, and $76.1\%$ of a given building having $2S$.
With some extremely-back-of-the-enveleope calculations and fiddling with normal distribution, I conjecture the average building to have about $2.1S$
Final calculation:
The dismantling time for a city about the size of New York and a process similar to the precarity of crane-disassembly would be
$$1000000\times2.1S\times\frac{3000M\times 1.12yr}{103S}\approx70000000Myr$$
So it would take $1$ person about $70$ million years to dismantle a large city safely and package it all up for transportation,
or $70$ years for $1$ million people.
Edit: This doesn't account for parts of the cities that fall outside the scope of that which isn't a part of a "building". However, most of such infrastructure has much to do with attached buildings. Regardless, I imagine a "fully-disassembled" city would look precisely like spare bits of infrastructure that falls exactly into such building-less category.
Edit 2: This does assume that a storey for a residential home is as complicated as a storey in the Empire State Building. Taking vica-versa, having one story of a residential home being made by $20$ people in $0.5$ years yeilds $\approx 0.025\frac{S}{Myr}$, which is actually less fast than the metric for the Empire State Building. So luckily this hindsight doesn't nearly change my conjecture about the estimate being off by a scale of magnitude or two.
Sources
http://www.centralplainscranes.com/FAQ_Crane_Operations.html
https://www.britannica.com/technology/skyscraper
https://www.cnn.com/2013/07/11/us/empire-state-building-fast-facts/index.html
https://www.visualcapitalist.com/100-tallest-buildings-in-new-york-city/
https://urbanomnibus.net/2016/05/how-many-row-houses-are-there-in-new-york-city/
https://ny.curbed.com/2018/4/23/17271092/manhattan-buildings-data-visualization-taylor-baldwin 87