If you're playing a class on your phone, you'll need a 4G connection. On average a 25 minute class will use 3-4GB, depending on the quality of your internet connection.
You have the option to change the streaming quality of a class to use less data, which is useful if you want to take a class outside or don't have access to wifi.
Start a class and tap the screen
Tap pause and you will see a menu with different quality options
Select Low Quality
Note: Scheduled group classes don't have the pause option, so to change the streaming quality, just tap the screen once the class has started, tap Auto in the top right corner, and then "Low Quality".
The amount of data this will use depends on the quality of your connection but on average, for a 25 minute class, the Low Quality option will use approx. 185MB.