The schedule option can be accessed from the action menu found on the Target Group, or target list (Hosts) screen. If a Query is specified, the scheduled the scan will first start an Enumeration task to perform the Query, and once finished will scan all accessible hosts in the target list. If no Controller is available, the Enumeration will wait indefinitely for one to become available. If you are performing a scan with Agents only (no Controller setup) then you will want to remove any Queries from the Target Group first.
From Target Group page:
From target list:
Scans can be scheduled for a target group on a daily, weekly, or monthly interval. Data must be entered in both the hour and minute boxes for the schedule scan button to become available. A scheduled scan can also be cleared from this screen if necessary.
After the scan has been scheduled the time and frequency information will be displayed on the target group card.