Estimating query time
If you go the route I suggested in your previous post I'm going to say &lt; 10 minutes.  But again, soooo much that isn't given will affect the wall time.  Is this a dedicated MySQL server or is other stuff going on?  Specifically things that will be affecting the disk i/o?  How fast are the disks? Are they SSD? Is it write through or write back cache?

In short there is no "explain" for operations like this that will give you a time value.

Don't forget to disable/enable keys during the modifications.

Report back and let us know the results.