Reference

https://www.fortinet.com/content/dam/fortinet/assets/data-sheets/Fortinet_Product_Matrix.pdf

Situation

# 更改Disk Log保存的天數，預設是七天
Change the disk log retention period (default is 7 days).

Solution

[1]
Log in Fortigate CMD

[2]
# 最多保存3650天
config log disk setting
  set maximum-log-age 3650