Prior to R80 the audit logs were kept forever with no automatic cleanup mechanism.
Starting in R80 whatever cleanup settings apply to the traffic logs also apply to the audit logs, although there is some conflicting documentation about whether only the audit log indexes are deleted as part of the cleanup process (which just slows down searches of the audit logs & change reports), or if the raw audit logs themselves are deleted too. I'm pretty sure it is the latter but not 100%.
Either way it is possible to configure custom retention values just for the audit logs by hand-editing the log_policy_extended.C file on your log server, please see: https://support.checkpoint.com/results/sk/sk117317
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