Access points checkin to Cloudtrax every five minutes to report usage data and check for any configuration updates. Checkins and replies for a 5xx firmware AP with ~10 users are both around 15KB. The checkin size can vary quite a bit based on number of users and how many distinct layer 7 traffic types are detected.
This would equate to ~288 checkins per-node each 24 hour day. This represents ~4.3MB of data per-node, so if you had a network of 10 nodes you'd be at ~43MB of data each 24 hour day, or ~1.3GB each month.