When I used to do scheduled scans, I always set them to run when the computer would be on but I wasn't going to be using it heavily (which would eat resources), aside from the on-demand scan it has all of the real time scanning to do.
- With a resident on-access antivirus like avast, the need for frequent on-demand scans is much depreciated. For the most part the on-demand scan is going to be scanning files that would be otherwise be dormant or inert. If they were active files then the on-access file system shield would be scanning them before being created, modified, opened or executed.
I used to have avast set to do a scheduled weekly Quick scan, set at a time and day that I know the computer will be on. But I have ceased this practice for some time now, based in the above.