In 1505 Michelangelo gave an estimate of five years to finish the tomb for Pope Julius II.

Due to small side projects like the painting of the Sistine Chapel ceiling it took around 40.

Failure to meet the deadline informed by the estimate meant that the scale of the project was massively reduced because: Pope Julius II had died prior to completion, there were changes requested by the customer (both Julius and his heirs), supply chain issues, contract renegotiations, labor disputes, shortages of qualified workers, and money running out due to the long duration of the project.

So, since 1505 at least?

The funny thing is that the pope isn't even interred there.