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.
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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.
