Of course the time for the scan will depend upon how many files (size) that you have on your computer, your OS and computer speed.
I have found that with 9 GIG of files scanning will increase in time by about 5 times if I scan archived files. Without scanning archived files scanning will take around 40-45 minutes. With scanning archived files 2½-3 hours.
Archived files are sometimes password protected as well, which won't be scanned anway as a result.
Btw, I usually DO NOT scan archived files. Maybe once every 4 months or so.
As far as whether it is a good idea to scan archived files as to whether or not they are vulnerable to viruses, that I'm not sure. I would guess that they are not as vulnerable for the one reason that they are not actively used, otherwise I'm not sure.