It is probably due to the system doing a self clean to many times. these printers need to be run every day to make the waste ink less. the more you print, the less time it has to cycle the ink to the waste tank.
I have found the setting in my environment of CMYK Clean every 16 hours and White every 12 hours to work better, but it depends on your humidity and temps in your space.
If your not familiar with these settings, press the print button (with no prints active) 8 times and an advanced setting comes up in your menu. go to the "Auto Head Clean " menu and you can adjust from there.
I am not all that pleased with this machine and how much waste it accumulates. But it's what i have and when it working right, it's a great printer. but damn it has it's bad days!
As far as your original question of how many prints, I really can't give you an exact number. I have done 200 shirts before on the same white cartridges, and was fine. On other occasions, I have done less due to the circulation/waste stealing my white ink...