I believe it has a 60 hour timer since last used and then it does a head clean purge which wastes a lot o fink. I try to remember to either use it or run a nozzle check to exercise it within the 60 hours. Nozzle check uses minimum ink and exercises all 10 colours. I don't think the resolution of quantity is reliable. I assumed it counted down how much was used since last replacement and used this to estimate remaining levels whcih would be easier and chaeper to implement, as it is in software, than a sensor in every tank. The sensors would then only need to give an out of ink warning.
