In many years of running my Epson roll printers, I use a general "rule of thumb" of between 1-2 ml ink per square foot of output.
My 9900 keeps track of the area printed and ink consumed for the last ten prints and when I've checked it, lighter prints take less ink and darker prints take more, so I'd say dgberg's good average is pretty close.
When I need to account for the ink on a particular project, I tend to error on the 2 ml figure since there are occasional cleanings that must be done and other maintenance that tends to consume the ink.
I've given up trying to track the actual "average" per square foot since each print is different.
