System Scan Takes a Very Long Time

I have a 500gb external drive and a 250gb drive internal to my imac.  When I do my system scans it takes a very long time.  I kick it off at midnight and at 8:00am it is still running.   Because I read somewhere that it must complete a full scan then the others will be shorter I am letting this one run to the end. Although I was confident I had done a full scan before, I am going to let this one run again. 

 

Anyone have any issues with this?  If it will always take this long, it will make the computer (or Norton) virtually unusable. 

 

Thanks