PMK weakens during development, presumably due to oxidation and depletion. This would depend on the amount of developer per film and the amount of air in the tank and agitation. For me, a 35mm film in a 250mL tank with only a little air in it seemed to exhaust the developer at a convenient contrast so that extra time made no practical difference. I discovered this when I tried for more contrast deliberately.
Yes, I am aware that 250mL is less than the recommended volume for 80sq.in of film. But presumably it's not less than the amount used by people doing rotary processing.