At least three times a week, 4 hours each session.
Normal kidneys excrete waste products continuously from the body. If any artificial method to excrete these waste products are employed ( like hemodialysis), that method should also be continuous. Conceptually therefore, hemodialysis should be continuous. However, that is not practical.
The minimum kidney functions that can keep an individual healthy is a GFR of 10 mL/min.
Hence minimum amount of dialysis should be at least equivalent to ( if not more than) a GFR of 10 mL/min.
It has been calculated to find that 4 hrs dialysis, three times a week is close to a GFR of 10mL/min.
For the same reason, hemodialysis will never reduce in frequency. Three times a week is minimum, not ideal.