FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Article Id 198581
The web filter quota option can be not found under web filter profile.

FortiGate will not list 'Category Usage Quota' in all type of FortiGate inspection modes.
Meaning FortiGate web filter supports this 'Category Usage Quota' feature only when FortiGate is running on the 'Proxy inspection' mode.

So if the FortiGate inspection mode has to be 'Proxy inspection' mode, which it can be changed from GUI:

Go to System -> Settings -> Inspection mode and switch from Flow to Proxy mode.

From v6.2,specify this 'Proxy inspection' mode only for specific web filter profile.
Which is named as 'Feature Set': switch from Flow to Proxy mode.

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