The app consumes 20 kB (kilobytes) per hour when idle.
Zello will use between 12kbps and 45 kbps when talking, depending on the connection type.
The audio encoder is chosen based on the connection type of both the sender and receiver.
If both are using WiFi, it will will use 45 kpbs. If either are using 2G, it'll switch to 12 kbps.
The active data rate is actually configurable in the range between 8 kbps and 40 kbps.
Average monthly data consumption for a single user ranges between 70MB to 100MB, although individual usage may vary.