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I run a full scan every Saturday morning because I'm doing other things then and so it's a convenient time. Thus I ran a full scan using the previous version during the morning; that took about 1.5 hours.
I downloaded 2.0 early Saturday evening and, because I was watching a film, I ran another customised "full" scan following that installation but kept half an eye on it; it had been running for 3.5 hours when I cancelled the run and most of that time was spent analysing files. On reflection perhaps it was selecting all the scans, including Archives and Rootkits, that was the key factor. I use a Windows 8.1 desktop PC but I suspect that is not significant; I also do regular file housekeeping so there is only a minimal number of temporary files.
Thanks for the replies.