It looks like that UTM doesn't count minutes, it simply immediately subtract user's choosen minutes from total allowed time.
In my example total allowed time for Facebook was 60 minutes, user choosed 20 when accessing the first time, so quota status showed 40 minutes remaining. After 20 minutes access was again blocked.
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Best regards
Martin
Sophos XGS 2100 @ Home | Sophos v19 Architect