Samsung SCX-4521F-XAA - Manual (Page 58)

Maintenance
8.4
Expected Cartridge Life
The life of the toner cartridge depends on the amount of toner
that print jobs require. When printing text at ISO 19752 5%
coverage, a new toner cartridge lasts an average of 3,000
pages. (The original toner cartridge supplied with the machine
lasts an average of 1,000 pages.) The actual number may be
different depending on the print density of the page you print.
If you print a lot of graphics, you may need to change the
cartridge more often.