Preferences → Anti-malware Engines Policy

From Engines page, open the Anti-Malware Engines tab, and click Setting Anti-Malware Engines:

This dialog allows to input:

• Number of Days since last Updated (in Days): If an engine hasn't been updated for a number of days that is more than this number, it will be identified as outdated. The default value is 14 days.

• Number of Allowed Outdated Engines: maximum of engines that are allowed to be Outdated. A scan cannot be triggered if number of Outdated engines exceeds the number assigned here. Default value is set to 7.

Alternatively, you can edit the same settings from Settings → Preferences: