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.
Presumably you don't work 7 days a week so your computer would be on for longer time frames, you could have your scheduled scans run sometime during the weekend.
There is also an option on the scheduled scan to wakeup the system run the scan and have the system go back into sleep mode when the scan is complete. Depending on your settings as a detection could wait for your input, if you don't have it set to repair and if that fails send to the virus chest. That said there have been some topics reporting the system doesn't go back into sleep mode, not sure if that is widespread or OS and system settings.