Alter auto-increment of table with 16 million rows Learned this the hard way. Need to alter a table's auto-increment with 16 million rows.

Now, it's taking quite sometime.

Any estimate how long it would take? Sorry.
It depends, could be an hour or so. Perhaps you may like to give percona toolkit a try; pt-online-schema-change.

pt-online-schema-change alters a table’s structure without blocking reads or writes.
Specify the database and table in the DSN. Do not use this tool before reading its
documentation and checking your backups carefully.


Source: http://www.percona.com/doc/percona-toolkit/2.2/pt-online-schema-change.html