VirusTotal Intelligence quotas are monthly. All quota consumption metrics are reset at 00:00 UTC on the 1st of the month.
To find your current quota usage click on the "Intelligence Administration" link on the menu under your username. Your usage numbers are on the intelligence tab.
Daily API quotas are reset at 00:00 UTC. If you have a monthly API quota, it is reset at 00:00 UTC on the 1st of the month.