ignore my mistake. This underestimates by a factor of about 12. Misread the source
I dismantled an old rusty 10m x 4m metal shed with only hand tools (A crowbar, socket set, ladder, ropes, hammer, chisel, and hacksaw) during the covid19 lockdown. It took 1 man about 4 days to do 40 square meters. Walls, roof, and cieling.
It was my first time doing something like this, and I was taking breaks and relaxing throughout the project.
According to https://www1.nyc.gov/assets/sirr/downloads/pdf/Ch4_Buildings_FINAL_singles.pdf , New York has 375 million square feet of floor space. In usable units that's 34838640 square metres.
Extrapolating my experience to all of New York city, thatll take 3483864 man days to dismantle all of New York city's buildings.
1000 people working in parallel could do it in 3483 days. Or a little under 10 years.
Now, there are lots of quibbles with this calculation that say it should be larger, steel frames are easier to dismantle than bricks, tall skyscrapers are harder to dismantle than single story sheds. But there are also quibbles that say it should be smaller, teamwork should speed things up, and they'd learn tricks to speed things up on the job. A religious cult also wouldnt stop for TV breaks. I'm handwaving here and saying they cancel each other out.
Now this will leave street, foundations, subway lines, signage, highways, water and power infrastructure, etc. This is very hard to estimate, I dont see cities spend more on common infrastructure than residents spend on buildings (based on relationship between tax and property cost), so I can guess an upper bound; I'd expect this would be no more than another 10 years.
Some earthworks are below sea level. Eg undersea tunnels, or foundations below sea level. The empire state building for example has 17m of foundations, 9m below sea level. This would need to be done by dismantling from the basement down, chiselling and hand-drilling away until they reach bedrock, then removing the side walls with the water proofing membrane. Hand pumps / windmills / steam powered pumps will have to be used to pump water out as they go.
Returning the materials to earth will be tricky without the city, you need power to melt steel down, etc. You may need to add a 5 years of breaking small concrete blocks down into gravel and land filling.
Add 5 years for unexpected difficulties that come up as a buffer, and add a support network of 500 to feed, shelter, bandage any injuries, transport materials, and create and maintain the hand tools.
That adds up to 30 years. Maybe leaving a bit of time left in their lives to dismantle the asphalt highways leading to a neighbouring city, or scatter plants or topsoil to start regrowing the landscape.
A religious sect with 1500 very motivated people could plausibly dismantle New York city in their lifetime.