Time taken to scan

I guess this is one of those “how long is a piece of string” questions, but is it unusual to expect a full, system scan to take over three days to complete on an iMac, plus back up drives? My first scan, (having just installed avast for Mac), is currently showing that it has taken over 45 hours of scanning time and is still going! Is this unusual? It appears to have completed scanning the iMac and is now grinding through my back up drives.
Is there a guide to what are the most effective files/folders/locations that should be scanned and which ones are safer to exclude so that if I do this scan again it doesn’t take so long?