If you can give some examples of the File name, location, malware name or reason for not being scanned ?
This gives us an idea if any action can be applied.
Files that can't be scanned isn't an indication it is infected/suspicious, so avast won't let you apply an action based only on it can't be scanned. Other instances are detections in memory, since these aren't physical files but memory blocks actions can't be applied.
Hopefully you can see there are legit reasons why an action can't be applied and why we need more information.
####
Re your first question on time frame for the Quick scan if a detection is made then it escalates the scan sensitivity. But that shouldn't happen every time, especially if no detection were made, your system spec, CPU & RAM will have an effect ?
On my XP system (spec in signature) my last quick scan scanned 6.5GB of data in just over three minutes.