Thorough, is very thorough, especially if you scan archives as well.
Generally I would run (thorough + archives) this only once, shortly after I had installed avast, to thoroughly check my system.
Archives are inert so I don't scan them on my regular scans, the reason, if you try to open an archive, standard shield scans it before it is opened.
My usual scans are Standard without archives, reason, because of the on-access protection of standard shield, scanning all created/modified files.
I do that scan weekly as part of my regular system maintenance and it takes just over 10 minutes for 5.5 GB of data.