I've done a couple of (more or less exactly) 48 hour MRs without leaving the airport.
YYC-DEN-LAS-SFO-EWR-MHT-EWR-SFO-LAS-SFO-YYC
This was the most torturous 48 hours of my life. I had a ridiculous same-day turn in MHT which I made thanks to a couple of favours from the pilots. Come to think of it, that was a same-minute turn!

Ran off one plane and on to another right as it was closing. Return EWR-SFO ended up getting routed through Canada and took 8 hours... in Y. Later, the RCC staff in SFO didn't even notice that I actually boarded a plane for LAS and came back.
YYC-YVR-SAN-SFO-EWR-BWI-EWR-SFO-SAN-YVR-YYC
I was more prepared for the torture trek ahead of me this time so it went much better. Being in AC C for YYC-YVR-SAN in both directions helped too.
LAX-IST-HKG-YVR-YYC
LAX to YYC via... the rest of the world. LAX-IST-HKG was pretty comfortable, I don't even want to think about how much travel time that all was. Probably around 48 hours including layover time, but they were long flights.