A multiple of coach fares can sometimes be misleading. 2x U fare, for example, could be a lot different from 2 x G fare. That said, I often use S fare as my benchmark, and I am always happy to pay 1.5 x S for a flight that is about 10 hours long. For very long 15 - 17+ hour flights I'll be happy to spend up to 2 x S -- but mostly for the higher priority on the upgrade list.