Hard to generalize of course, but my gut feeling (based on my company’s experiment for 13 weeks) is that it’s less than 10%. Potentially increasing longer-term productivity. Emphasis on long-term because it might seem like a drop in the short term. Longer term job satisfaction and general well being can increase, focus and just being able to recharge more effectively. Yes, completely anecdotal. FWIW My company since decided to switch to 4-days all year in 2021.