Back in the 19th century, Charles Bargue observed the quality of draughtsmanship at the French academy was declining. He traced the cause to the lack of classical models, and created a series of nearly 200 lithographs to be copied by students.
The drawing course he created consisted of the following:
* Classical models of sculptures, like the Belvedere Torso, the head of Dante, and various arms, legs, hand and feet.
* Series of academical figure drawings in a simplified linear style.
* Master drawings intended to be copied as accurately as possible by the students.
Having re-copied all the litographs the student was ready to draw from life. Van Gogh copied the entire set, so that's 200 large drawings meticulously copied, and if you assume 5 days per copy it totals 1000 days of drawing.