Whilst I don't work for avast I suspect that, before the time/percentage done/remaining can be displayed it would have to analyse the data that you want to scan.
If infected files are found then the avast scan goes to a much higher degree of sensitivity/thoroughness - almost paranoid mode. This would not only take longer it could also mean that more data is going to be scanned, hence the revised figure of how much has been scanned.
Personally - 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.