Marx has some concept of "socially necessary" labor, but it doesn't really make sense because it wants to treat skill as some kind of simple multiplier as if we can assume that a modern doctor's labor is some simple multiple of a random medieval peasant's labor in diagnosing an illness, which is just a silly and oversimplified model.
Honestly it reminds me of like Marx's non-rigorous "eh whatever" approach to dividing by zero in his mathematical work. While one can define division by zero in non-standard analysis, that involves some actual rigor in how your treat it according to your definitions, not just shrugging your shoulders and doing it.
