One cartridge contains enough reagents to run approximately 96 measurements which is equivilent to 4 chip boxes. The percentage remaining is indicated in the top right of the instrument display. Changing the cartridge takes a few minutes and does not require any special tools.
The exact protocol may vary depending on the software version of your instrument — check the user manual for full instructions on how to perform the change.
Reagent cartridges are disposable and should not be re-filled.