Hey Joe
Flew earlier this year to Buenos Aries with BA from LHR. Direct flight - 15hrs 55mins outbound, 15hrs inbound. Not sure if it still is - but used to be BA's longest flight (timewise)
747-400 operating the route at the time of travelling. Booked a couple of seats in Club World, for me & Mrs HP as BA econ IMO, is far too squashy for anything over 5hrs.
Wonderful place - must go again some time. Hope you enjoy it too
All the best
HPC