If learning durations were log-normally distributed, how would people be able to graduate from universities and finish studies, mostly in time? Or accomplish anything substantial in their limited time span?

I agree that distribution of most human tasks' duration is skewed (not necessarily distributed log-normally), but these tasks can still have a reasonable upper bound for completion. The success is not binary. Like in grading, we need to accept that some projects will get an A, and some will get only B or C, and it's still OK. Some may fail.