How long did some famous and valuable art pieces, like paintings or sculptures, take to make? Did any famous artists record their time spent on various pieces and divulge them to the public?

Answer 1

That depends. Some artists, like Adolphe Bouguereau, could turn out a painting every day. While this is not as common among works with a lasting impact, many of these artists enjoyed immense success in their days. On the other hand, Cézanne never considered a number of his most important paintings to be finished. In some cases, you can even see unpainted white spots on the canvas.

As for method, again, there's a huge variety. One thing you could look at if you want to see a painter in action is Henri Matisse's Bathers by a River, which took eight years to create, and went through a number of drastic revisions painted one on top of the other. Through X-radiography, art historians have been able to reconstruct the development of the painting as recorded on the canvas. This gallery has the full story: https://archive.artic.edu/matisseinteractive/p0.html