As an experiment I developed a roll of 35mm in 250mL of standard PMK in a tank without much air (air or at least oxygen is part of the chemistry of PMK which is one reason why results seem to vary) for i think my standard 12min, not much agitation. Good negs as usual. Then I used the same used developer immediately on another exposed waste roll and got nothing, not the faintest density, not edge numbers, absolutely nothing even viewed very carefully. I concluded that my 12min or so was close to development to exhaustion, at least for this quantity and a full roll.
I concluded that a larger quantity of developer would be required for longer development than around 12min.