I needed to get the Uptime/Duration of a particular VPN tunnel this week. It was for a client with multiple VPN tunnels that was having problems with just one.
Solution
Option 1 via Command Line
1. Connect to to the firewall > Go to enable mode and use the following command, replace 123.123.123.123 with the IP of your VPN endpoint.
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PetesASA>
PetesASA> enable
Password: ********
PetesASA# show vpn-sessiondb l2l filter name 123.123.123.123 | incl Duration
Duration : 0h:08m:26s <<<<<<<
PetesASA#
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If you want a LOT MORE information use the following command;
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PetesASA# show vpn-sessiondb detail l2l filter name 123.123.123.123