I have printed about 100 prints of varying sizes from 8x10 to 16x20, almost exclusively B&W without any toning (maybe the equivalent of 6 16x20 color prints have been made).
These have been using Photo black and no Matte black ink. I've noticed that the Photo Black , Light Black and Light Light Black inks are down appropriately with remaining % of (51, 29, 49). The Matte Black has 87% remaining so 13% to initially charge the lines.
However, the color inks are also down considerably with % left of Cyan 66, Vivid Magenta 66, Light Cyan 52, Vivd Light Magenta 47, and Yellow 65.
Is there color used in what I believe to be pure B&W prints? I don't think turning off and on would waste ink as the Matte Black is still relatively full.