When he got picked up and taken off to school.
Stefano concluded his in-school education, was proclaimed a Bachelor, and sent off into the world for his Dissertation quest
It really is not set where or exactly when this occurs, but sometime during Stefano's first year as a Bachelor, and between Jack meeting the Ravens and returning back after that winter, they ran into each other. Stefano regrets whatever ends up happening, Jack does not trust him, and I am not sure yet if Rose was involved or not. But something happened.
this was his time as a Bachelor, the majority of which was devoted to his Dissertation quest
Stefano's dissertation quest most likely culminated in some dusty planar barrow somewhere under a mountain, and was most definitely dirty, ugly, but ultimately a success. After this he returned to his base camp, after which he got yoinked back to present his findings and discuss his actions during his 6-year quest, at which point he was proclaimed a Master.
His time spent back to more academic studies and trying his hand as a tutor to nobles.
The very poorly executed "trip" that accidentally scoops up ER.