Four days
61 British companies, some two thousand nine hundred people, six months. Pay stayed the same, but the week got one day shorter.
Burnout fell by 71 per cent. Sick leave by 65. Resignations by 57.
Revenue grew by one and a half per cent.
Of the 61 companies, 56 kept the system.
The study says nothing about the other 5. They went back to work.
Source and further reading: University of Cambridge: the largest four-day working week trial to date