I have over 1.6 Million files on my hard drive. But avast doesn't even hit 1% in 7 hours (I end up cancelling it). However I did a MalwareBytes scan yesterday and that took roughly 7 hours and worked.
When you run Full System Scan for the first time, it usually takes long time because it stores data for the already scanned files in its persistence cache. Regarding avast not hitting 1% for long minutes/hours, I take it as a bug as it is same here. But the behaviour will not show the next time you run the scan because it does not scan twice if the file had already been scanned and not modified after the first scan.
Here, avast takes around 2 hours for first FSC and MBAM takes few minutes more. So, if MBAM took that much time in your case, then avast would take a little less. Reset your settings to default again (Settings> General > Settings Backup> Restore Settings) and run the scan once again.
Be patient for that much files especially if most of your files are huge as that significantly increases time.