Thankfully, not much. The app’s main data traffic source would be the map (Google Maps), which according to a quick search, consumes around 40MB/hour, in the worst-case scenario. In fact, it’s almost 0 if you’re always on the same route because the data is already stored the first time you opened the route. One tip to save a lot of data is to open the route, at least for the first time, using a Wi-Fi network.
Don’t hesitate to message us at firstname.lastname@example.org if you have more questions. Have a nice day!