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.