If after eight weeks a junior engineer is still toiling on their story, I'd ask why someone more senior didn't get involved.

There are lots of reasons - maybe the senior engineers are overburdened with other work (or don't care), maybe the project manager or team lead wasn't asking if the junior needed help, or maybe the junior was lying about their progress.

Either way, a story that goes for eight weeks feels excessive. Much, to your point, taking eight weeks to figure out that there was a bad part feels excessive. My counterpoint is that teams don't typically operate like labs. In a college lab, the objective is for you, specifically, to succeed. In an engineering team, the objective is for the entire team to succeed. That means the more senior engineers are expected to help the more junior engineers. They might directly coach, or they might write better documentation. I don't believe that dynamic is present in a lab setting.