Due to the number of variables that lead to data usage in ForeFlight, we can't provide you with a specific amount of data usage to expect. However, Apple has provided a means of seeing cellular data usage by app. To do so, open the devices Settings app and tap on the CELLULAR line.
Image 1: Select the CELLULAR line
Scroll down and you will see an alphabetical listing of the apps you have that use cellular data. Search for ForeFlight and the usage will be displayed.
Image 2: Scroll to where the apps are showing and view ForeFlight usage.
If you wish to see the usage starting from zero usage, then scroll all the way down to the bottom of that page and select RESET STATISTICS. Once that is done, you will see all the cellular data usage by ForeFlight going forward.