I have found a quick and easy way that works for me is to search for the subnet assigned to Client VPN.
For example, if you have 192.168.27.0/24 assigned to Client VPN connections...just search for '.27.' in network connections and it will show you all of the connections for the given time period.
If you drill down into the client details, it will show you the last time it connected and when it disconnected (although, my disconnects are hit and miss, think it may be if they truly disconnect vs. a computer timeout or shutdown).
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