One of the things about the mission endurance limit for the E-7 Wedgetail, and the P-8A Poseidon for that matter, is that there is a limiting factor other than 'just' fuel, namely another liquid carried aboard IIRC, that cannot be replenished in flight.
I believe its engine oil, which also restricts a few other jet types. Here's a reference that quotes a Boeing manager of the P-8 programme...
"The P-8 has aerial refueling capability, though as-yet unused, to allow 22 hours of endurance (at which point, engine oil is largely consumed)."
Boeing's P-8 program viewed as "model" defense procurement - Leeham News and Analysis