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 was built in 6 months to separate the uranium.

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.

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.