With the number of resident providers running this on-demand scan is less crucial, but I still run it manually one per week (as part of my regular system maintenance), on Standard sensitivity and without archives. For me I feel that is enough, others suggest monthly and a thorough sensitivity and with archives, personally I feel this is too long.
Thorough is also by its design very thorough (it scans all files) and perhaps a little overkill for routine use, were a Standard scan without archives should be adequate. Archive (zip, rar, etc.) files are by their nature are inert, you need to extract the files and then you have to run them to be a threat. Long before that happens avast's Standard Shield should have scanned them and before an executable is run that is scanned.
I have only ever done a Through Scan with Archives once shortly after installation just to ensure a clean start state, but with XP for example avast will do a boot-time scan after installation if you select it, this I believe will be quicker and reasonably effective. Like everything in life things are a compromise.
