Good question.
Take a look at the video of ÖKO EUROPE's mobile application: ÖKO Counter, available free of charge on Android and iPhone.
Using the app is the ideal way to avoid over-consumption of filter and to know exactly when the filter is due for renewal. The latest version of the app includes a notification system so we don't forget to increment our daily consumption.
The filter lasts 400 liters and costs €29, which represents a cost per liter for healthy water of €0.0725.