You never run out of toner in a cartridge. Just give it a shake for another lease on life. It’s like Zeno’s paradox. You just have to shake it increasingly often.
In theory. In practice, when you need to shake it more often than once per page, it becomes unusable. In physical universe, once you need to shake it more than once per scanline (perhaphs 1/600inch), you start losing resolution (classical version of Heisenberg uncertainty principle).
The laser printer Planck limit is rather large, and not constant.