SIN-PEK, 2,780 mi
The farthest I think possible on scheduled service on one flight would be the CZ354 pre-pandemic, SIN-CAN-SHE and the SIN-TAO-SHE measuring at 3,027 and 3,028 mi respectively, on more than one flight, maybe SIN-CAN-URC-KHG? CZ3040 and the CZ dreamliner service between CAN and KHG.