How often should virus definition downloads and system virus scans be completed?

Daily is often the ideal or automatic frequency for definition updates, but the question asks about *scans* as well, and "weekly" is a common general recommendation for full system scans.

Monthly updates and scans are too infrequent given the rapid pace of new malware development and could leave systems vulnerable for extended periods.

While daily updates are ideal and often automatic, a weekly schedule for virus definition downloads and full system scans is generally considered a minimum best practice to ensure systems are protected against newly emerging threats and to detect any malware that might have bypassed real-time protection.

Yearly updates and scans are dangerously infrequent and would provide virtually no effective protection against current threats.