PC World did an investigation of how much ink is left in an inkjet cartridge when the printer claims it's empty. In the case of the Canon MP610 (the same printer I have) the printer stopped when there was still 24% of the ink remaining. (via Lifehacker)
Happily there apparently is an override for this, at least with many recent Canon printers:
1. Turn off the printer
2. Hold the Resume Button (red circle in the triangle), add a printer (click “Power”) an indicator light green.
3. Hold the Power Button, release the Resume Button.
4. Not releasing the Power Button, double click “Resume” and release Both Buttons.
5. Click “Resume” for Four times
6. Once you click “Power”, to confirm the selected action (reset counter absorber). To disable the printer “Power” button to click again.
A quick Google should hopefully find workarounds for other brands of printer too.