The Toner cartridge yield is up to 6,500 pages when printing at 5% coverage on A4/Letter sized paper. When you see the Toner Low error message you should order a new toner cartridge as the toner will run-out soon.
For use with the HL-1850, HL-1870N,HL-5040, HL-5050, HL-5070, MFC-8820D, DCP-8020, DCP-8025D