So it looks like you asked this before without any answers, so I will stick my neck out on this one: My suspicion is that your friend was getting confused with the claim that medieval peasants had more days off than modern, industrial/post-industrial workers. This as come up in the past with the basic response being that the claim itself rests on rather spurious reasoning. Partially this is a weakness of source material, partially this is the difficulty of defining "work". Should we count only the time sent working on their lord's field, which is the most directly comparable to modern work? That seems wrong, because it is hardly the only thing a given peasant would need to do in a given week, but then it becomes progressively more difficult to define "work". Working in their own fields? Repairing their tools? Working around the house? In that case, if I spend my Sunday working in my garden should I count those houses towards my work week?

Anyway, this is all leading up to saying that I cannot even begin to think of the methodology for making that statement. The actual lives of Roman slaves, and particularly how they thought about and experienced their lives, is essentially a closed book. Even if "Roman slave" was a unified category with a singular lived experience, which I have trouble accepting given the differences in lifeworlds between a captured German made to work in the fields and a Greek tutor, we simply do not know how their day to day life went.