Estimating query time Is it possible in MySQL to know beforehand query execution time? I mean how long it will take to execute the query.

Generally its worthwhile to know how long any query can take to execute.

At the moment, problem at hand is Converting CHAR to BIT (1.2GB Table size, MyISAM, indexes not on the columns I want to change, 3GB RAM)
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.