I believe the point in question is not he maximum limit of endurance due to fuel capacity - but because of O2 capacity and/or engine oil capacity - assuming the latter being, as with all jet engines, a lot all loss system.
I can remember the major sucking if thumbs as too the maximum engine oil endurance of the F4 when planning the initial deployment to the Falklands and if they should be routed close to the Douth American coast in case of problems - the problem being, IIRC, that no trials had ever been done and nobody knew the loss rate - the tank just being filled up when the level got too low. .
Reaching back, did they put one up on a test with a tanker and just get i5 to fly round and round the UK? For hours and hours?
Eventually they just went for it and they arrived with plenty to spare.