Jim,
Yep. I'll give you some ranges because it really depends on the Weather, TFRs, and Nexrad activity. Basically, the iFly GPS can use as low as approximately 2mb of data per hour of continuous use, and as high as 10mb of data per hour. The higher range is if you are on Weather Mode as it is pulling NexRad data more often. That 10mb would also be if there was a huge amount of nexrad across the country. It would be toward the lower end if you used Sectional, or other mode than Weather.
These numbers are when the app in communicating to the WiFi, 3G or LTE provider and the App is in the active foreground of the device. If it is in the background "multi-task mode" of the device, it is not using any data.
If you want to limit the data usage, just uncheck the "Attempt Weather Update during flight", located in the Setup menu. This will stop data usage when in flight and on the data network. Also keep in mind, if you launch the App on a Wi-Fi network, it will pull all the data in and cache it on the device for several hours. I hope this helps.