How much ink on average does an office use?

Depending on the printer model, toners have a capacity varying between 2000-10000 sheets with 5% coverage. This number is usually overstated by the manufacturers, so in practice is around 70% of the declared capacity. Given the average number of sheets that an office employee prints out (around 10000), typically he/she would need between 1-5 toners per year.

Sources:
http://www.pcstats.com/articleview.cfm?articleID=869 http://pc.pcconnection.com/1/1/74341-xerox-printer-division-black-high-capacity-toner-cartridge-phaser-6100-106r00684.html http://www.instaoffice.com/black-high-capacity-toner-cartridge.xer106r01147.0.7.htm