DIAGNOSIS Brother printers are conservative with their toner level estimates. The 'Toner Low' message is based on page count since the last reset, not a direct measurement. It comes on early to give you time to order a new one, so yes, you can often print hundreds more pages.
FIX You can usually ignore it until print quality suffers (faded text, streaks). To temporarily clear the message from the display for the current power cycle, open and close the front cover. For a longer-term reset (if you've replaced the toner or shaken it), you can go to Menu > Initial Setup > Reset Settings > Toner Life Reset.
IF THAT DIDN'T WORK If the message returns instantly after a reset or the printer stops printing despite okay quality, the toner cartridge's internal chip might be faulty. A non-genuine or remanufactured cartridge can also cause this.
HEADS UP Don't shake the cartridge like a can of paint as a long-term fix. You'll get a few extra pages, but you risk leaking fine toner powder inside the printer, which can lead to other problems like sensor contamination.