they do some squirrely stuff to try to get you to buy a new toner cartridge early
My Brother is newer than yours (the cheapest one I could get that prints on both sides of the paper), and has a setting to toggle how it behaves when toner is low.
The default is to pause printing until you replace the toner - honestly that's not entirely wrong. Having the printer run out of toner half way through an important print job could be a disaster.
The alternative mode is to just show a "low toner" warning badge whenever you print a document. That's what I use, but I also check if it printed properly before closing the document which a lot of people don't do. It looks like this:
As far as I know it's just a simple counter - how many pages have you printed since it was replaced. Obviously that's never going to be particularly accurate.