@KarlHalpin,
I personally recommend that clients keep a script running that can record the number of active users. The basis of that is simply '
https://<firewall>/api/?type=op&cmd=<show><global-protect-gateway><statistics></statistics></global-protect-gateway></show>&key=<key>' and you would want to parse the return to ['response']['result']['TotalCurrentUsers'] to return the value of active users. This value can then be written in any format that you want whether that be to a log file of some sort, a database, or whatever else you decide.
If you're looking solely at historical logs instead of those moving forward, the logout events will give you the duration in seconds that you can use to calculate who was connected when. Obviously this method would present a large amount of overhead and review to get it broken down to a specific day or further than that, but it is certainly possible with enough time investment.
Just realized how poorly that example presented.
https://<firewall>/api/?type=op&cmd=<show><global-protect-gateway><statistics></statistics></global-protect-gateway></show>&key=<key>
# Replace <firewall> with your own firewall FQDN/IP #
# Replace <key> with your API key #
... View more