What is the expected lifespan of the drum unit on the DCP-B7500D?

The drum unit (model DR-B021 for India, or DR-B022 for Indonesia and Vietnam) has an approximate life of 12,000 pages based on 1 page per job using A4/Letter simplex pages. Note that the actual number of pages may be affected by factors including media type and media size. When the drum needs replacing, an error message will appear on the machine's control panel or in the Status Monitor. (Page 15)