How much effort are you putting in?

The pyramids were kind of like FDRs New Deal. When there wasn’t work to be done, the workers worked on the pyramids and still got paid.

If there was work to be done, they didn’t work on the pyramid.

Skyscrapers typically take 2-5 years to build. I’m going to hazard a guess that a pyramid like the Great Pyramid of Giza would take more like 5-10 years.

There’s a lot of variables to this, especially the location and the source (quarry) and transportation of the high-quality limestone that you would need. You would also want to use some granite and other stone materials if you’re trying to match what the ancient Egyptians did. (If you’re allowed to use modern concrete and steel, it could change things dramatically.)

The Great Pyramid is about 147m tall and 230m wide along each side. Assuming it’s mostly solid stone, You would need roughly 2.5 million cubic meters of material.

Preparing and grading the site would go much faster with bulldozers and graders and the like, but is still a time-consuming process. Quarrying the limestone would also go much faster than the ancients because we have explosives, hydraulics, etc. But there’s still a lot of precision cutting and fitting to be done. Cranes would make placing the stones go faster but again, still time-consuming.

CAD software would also make the whole project faster and easier (and would further highlight how ingenious the ancient people were with this accomplishment). Experts already know a lot about the pyramids, but I bet if they built one they’d learn even more.

My wild guess is: if you marshaled enough resources and put a highly competent person in charge of it, you could get it done in 5 years.

To make something that looks exactly like a pyramid, I imagine the easiest thing to do would be to fabricate large stone blocks and stack them the same way the pyramids were built. Cranes could make short work of the stacking, and if you’re willing to spend more, you could make the blocks out of lighter materials like fiberglass and foam. Even with natural stone, you could have multiple cranes and multiple crews to be done in like a month. Fabrication and delivery would also depend on the availability of the materials, so that might take another six weeks if you’re willing to spend the money.

If it were a standard construction project, you might budget for two years to create the drawings, get permits, bid out the job to contractors, prep the site, build the dang thing, and get final inspections. How much it would cost would also depend on materials selected and where it was built. Labor rates, land value, material costs, all of that varies by location and even the seasons.

Budget, Quality, Speed. These are the three corners of the universal priority triangle. If you want it fast and good, it’s going to cost a lot of money. Fast and cheap will be low quality. Good and cheap will take a long time (maybe forever).

The Louvre Pyramid took five years to complete. Designed in '84 and completed in 1989.

The Walter Pyramid took two years to complete. It’s 18 stories tall, and can seat 6,000 people. Each side of the perimeter of Walter Pyramid measures 345 feet (105 m), making it a mathematically true pyramid. It is one of only four true pyramid-style buildings in the United States, the others being the Summum Pyramid in Salt Lake City, Utah, Luxor Las Vegas in Las Vegas, Nevada, and the Memphis Pyramid in Memphis, Tennessee.

Pyramids are pretty simple. What you really want are today’s architectural wonders.

The Petronas Twin Towers are a pair of 88-storey skyscrapers standing at 451.9 m (1,483 ft) in Kuala Lumpur, Malaysia’s capital. They are the world’s tallest twin towers and were the tallest pair of buildings in the world after their construction in 2004. Both buildings are linked by a 2-story skybridge at their 41st and 42nd floors and the bridge is 170 m (558 ft) tall, which makes it the world’s highest 2-story sky bridge.

The Washington Monument is the world’s tallest stone structure, and the world’s tallest obelisk. It’s made of granite, bluestone gneis, and marble. It had a staggered construction, and took around 10 years to complete.

The Sydney Opera House took fourteen years to complete. Considered to be a modern masterpiece, it was the result of a competition and was opened by Queen Elizabeth II. It has also been featured in Star Trek as a hat.

The Hoover Dam is a concrete arch-gravity dam in the Black Canyon of the Colorado River, on the border between the U.S. states of Nevada and Arizona. Constructed between 1931 and 1936, during the Great Depression, it was dedicated on September 30, 1935, by President Franklin D. Roosevelt. Its construction was the result of a massive effort involving thousands of workers, and cost over 100 lives.

And so on and so forth

The Manhattan project took 2.5 years. It involved mining 18.8 million pounds of uranium. 10 Cyclotron factories of 20 cyclotrons for 22,000 workers were built in 6 months to separate the uranium.

It boils down to collecting all the building equipment from around the world and sending it to where you want to build the pyramid.

50,000 people cutting stone while 50,000 heavy construction equipment operators move the stones would take 2 months.

So it’s really down to resources. If it was a Manhattan Project level of resources thrown at building a Pyramid, I’d wild guess 3 months.