Up to how many hours a white collar worker can efficiently work out of 8 daily working hours and what should be the perfect working time schedule?

maximum5

8 ساعات عمل يوميا مناسب جدا كحد اقصى لجسمك عليك حقا والحياة ليست كلها عمل فقط فهنالك ترفيه والتزامات عائلية ومجتمعية وغير ذلك من امور

Eight hours per day but u can let them use the flex time option

Every worker can work efficiently upto8 hours provide the working hours and rest hours are distributed idealy, for example i prefer the timings to be:7:30 A.M.
to12:30 P.M..
Followed by a lunch and prayer break from12:30 P.M.
to01:30 P.M.
Finally01:30 P.M to04:30 P.M.

For me time is not a constraint as long as it is being utilised to achieve the desired results or exceed expectations

Its depends on person whether she/he is in a white collar or black collar job, actually its own self who decides to adhere to time. Duty hour fully depends on nature of business.