You can't use scan duration to determine anything about the persistent cache.
Unless you have something that you can compare with, then again you can compare 5.0.677 (there is no 5.1.677), e.g. total files scanned and total GB scanned for the scans under 5.0.677 to what is being scanned now.
Whilst avast may also have added more packers to the latest 5.1 version, if you selected all packers in the full system scan then that would also make a difference.