I just wonder how often will Tor send a packet when the circuit is idle (when you launch Tor and do not visit any website)? I think Tor will regularly send a cell to keep the circuit alive.
I did not find any solid number in Tor's manual.
Unless you've modified the KeepalivePeriod in your torrc, it should be using the default value, which is 300 seconds (5 minutes) according to the documentation.
