Hi,
It is currently not possible to set a time for Automatic Maintenance.
Avast Cleanup Premium scans your PC every 7 days, and Automatic Maintenance only runs if the scan reveals one of the following issues:
100 MB or more of cleanable space on your system
100 or more broken registry items
100 or more broken shortcuts
100 or more tracking cookies
Every time you run a scan for browser items it will have to close any open browsers before it can fix any issues.
Perhaps you can consider disabling the browser-related items under the Automatic Maintenance settings and perform a manual scan of those instead.
https://support.avast.com/en-ww/article/40/#idt_0760