Mind you, I regularly fly 14+ trips on a 345 with full payload and that feels like a bloody long way.
With a lighter load it will go for 18hours. Is that long enough for you ?
For what purpose? Certainly, few customers need more... One thing which people are trying hard to do, and cannot, is find a plane capable of flying from Sydney to London, reliably, with a commercial payload.
What is the endurance of 4 pilots? And how many are needed for longer flights - 5? 6?