The longest I've been on was HKG-NRT-HKG-LHR-ACC-LHR-DXB (in F) over 3 days as part of a RTW ticket.
Also on that same RTW ticket I did AUS-DFW-LGA/JFK-SCL-LAX-JFK-SYD-CNS all in one go (actually I think that was longer) but I left the airport in SCL for the day.