Currently a device with 1 service (IE ssh) and bulk scripting will use the following amount of bandwidth when idle:
.02 Mbyte per hr, .49 Mbyte per day, 14 Mbyte per month
Each extra service you add to a device will consume an extra:
.01 Mbyte per hr, .24 Mbyte per day, 7 Mbyte per month.
An idle connected service (P2P or proxy) will use the following amount of data:
.065 Mbyte per hr.
This is not including Data transferred over the connection.
Cellular connected devices may not want to leave idle proxies or P2P connections running.