The short answer is, it depends. When Propeller is running in the background on your smartphone or tablet, it's occasionally checking to see if the sensor is trying to send data and then sending that information back to our server. This action does not require much data at all. Your data usage also depends on how frequently you're opening the app to check your data and add additional information, just like with any other app.
You can set Propeller to only try to send data when your phone is connected to Wi-Fi in your phone settings.
If you're concerned, check the data usage information on your smartphone or tablet. If you're unsure how to do this, contact your mobile provider or smartphone help desk.
Are you curious about how much data you’ll need to download Propeller? See this article for more information.