When you have detected an infection that can't be dealt with in the normal windows mode, like file in use or is protected in some way. Or you suspect a problem or after a detection.
The difference is simple the scan is done before windows starts, so there should be no isue with file in use or protected, etc.
I can't say how long it would take but it would be considerably less than a thorough scan with archives, it may even be quicker than the quick scan but systems are different and te best way to check is test it. Though after you installed avast it is normal to ask if the user wants to do a boot-time scan. If you accepted that offer then that should be a reasonable idea of the duration.
