The Zello app consumes 20 kB (kilobytes) per hour when idle (160 kilobits). 
Zello will use between 12kbps and 45 kbps (kilobits per second, or 1.5 to 5.625 kilobytes per second) when talking, depending on the connection type.
The audio encoder bitrate is chosen based on the connection type of both the device sending the message and the device(s) receiving the message. If both users are on WiFi, it will use 45 kbps. If either is using 2G, it'll switch to 12 kbps.

If you are worried about data consumption, you can change the bitrate used during sending/receiving messages by going to Settings in the management console. From the settings menu, go to iOS or Android and scroll down to Voice Quality. (Android and iOS settings must be configured separately, configuring the setting for one operating system will not impact the settings for the other.) You can then change the Bitrate setting to limit the rate in order to conserve data. Please note that while it will conserve data, lowering the bitrate may also negatively impact audio quality. 

When Bitrate is set to Auto, Average monthly data consumption for a single user ranges between 70MB to 100MB, although individual usage may vary.

Image Resizing

Large image files will always be resized within the Zello app to be no larger than 1,280x1,280 pixels and 300 KB. When you are on 2G cellular connection, images will automatically get resized to be no bigger than 320x320 pixels and 100 KB.