I suppose that would depend on how much traffic your receiver picks up.
I'm seeing 41 aircraft right now (it's the middle of the night...obviously this would be more during the day) and the aircraft.json file is about 12KB. That is downloaded every second. So that's 720KB every minute, or about 10MB in 15 minutes, or 40MB/hour.
During the day I see at least 150 aircraft at any given second during peak hours. Now we're at about 45KB/sec, or 2.7MB/min; or 162MB/hour.
It can certainly add up fast.
But the file you reference is on the Pi itself, I believe. My Pi stays at home connected to WiFi. I am asking how much data the device you type xxx.xxx.xxx.xxx:8080 into is using while displaying the live updating map data. I am looking to roam around and use cellular data to display the live map screen with the planes on my iPad potentially for several hours at a time. The Pi itself always stays at home.