I think the workweek probably should be shortened and that a 35 or even 30 hour workweek should probably be considered full time, but I extremely doubt the claims that wage/salary workers are doing useful work for only the minimal period of time that certain communists claim.
There’s a tremendous range, is the thing. There are a lot of wage workers who have task-critical jobs that require constant input of effort. There are also a lot of wage workers who could disappear for weeks or months on end without anyone actually noticing the effect on any product or process. I have yet to find any two people who have very similar estimates of the ratio between these two populations.