The cpview (via the -t historical mode) and sar commands automatically maintain 30 days of historical data. Further back than that you will need to rely on logfiles assuming they have not already rolled off such as:
/var/log/messages*
/var/log/spike_detective/spike_detective.log
If you have the monitoring blade enabled with the right options on your gateway you can also try a System Counters...Firewall History or System Counters...System History report and set the timeframe for "since system installed". You may be able to also construct a custom SmartView Monitor historical report that goes back more than 30 days. Beyond that there isn't much else to look at if you need to go back more than 30 days unless you have SmartEvent.
Gaia 4.18 (R82) Immersion Tips, Tricks, & Best Practices
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