If you own a laser printer you will know the telltale signs of an empty toner cartridge. Either the page starts to get streaky, or, on more advanced printers, a message tells you that you need to replace your toner cartridge.
When that happens, an old trick is to remove the cartridge, then, holding it level, give it a few firm slaps to dislodge toner that has stuck to the sides. You'll be amazed how many ‘bonus' pages you'll get! You can even try this a second time, but results won't be as good.