TN-3145 Up to 3,500 pages @ 5% A4. When you see the Toner Low error message, the toner cartridge is almost empty. You should order a new toner at this time to be ready for when the toner cartridge is out of toner.
The toner is rated when printing on single sided A4/Letter sized paper at 5% page coverage and with the print density level set to 0. For best performance, use only genuine Brother supplies.