Should note that doing this can damage the printer. The toner particles that don't stick keep getting recycled back into the cartridge until eventually clumps of that crud are most of what's in it, and you're still trying to use more and more voltage to pull it out onto the paper... possibly getting it stuck somewhere else in the printer, damaging the drum, clogging a fan, or any number of risks.

If you're 5 times past the rated capacity of the cartridge and it's printing light, instead of shaking the crud around and putting it back in, just buy another cartridge... unless you WANT to destroy your in-laws' printer and burn their house down.