Open front door of the printer
Take the cartridge out of the printer.
Press green tab on left side of the cartridge and take the toner unit out.
On right side of the printer you’ll find a clear plastic lens. A laser beam shines through this lens and detected by a sensor on the other side. In case sufficient toner is present in the cartridge the beam is blocked and can’t hit the sensor. However if it successfully passes through the lens it means that toner is near to end. In this condition printer will display a message to replace the toner cartridge even some amount of toner is still there. You can consume the remaining toner if you prevent the laser beam to reach to the detector. Masking tape, a piece of paper or any other hard material can be used for this purpose. Some youtube videos suggest that you might print more than 3000 prints on MFC-7860DW, TN-450bcartridge and other models.