How often should a website be maintained?

The maintenance cycle for the most part depends on your risk profile. 

Low Risk Profile Maintenance Cycle
If the website has a low risk profile, then an annual maintenance plan is likely to be sufficient. However, given the ever-increasing risks associated with malware in today’s world, we recommend the website be updated a minimum of every 6 months, or quarterly if you can afford it, just to be on the safe side and protect your investment in your website.

Medium Risk Profile Maintenance Cycle
If the website has a medium risk profile, then a monthly or quarterly maintenance plan is recommended.

High Risk Profile Maintenance Cycle
If the website has a high risk profile, then a monthly or continuous maintenance plan is required. 

Continuous relates to the idea that throughout the month the website is inspected to see what parts of the website (server PHP, CMS, Theme or Plugins) are in need of an update and corresponding backup.