It will take hours.
All writes to the table will be blocked for the entire time.
A temporary InnoDB will be created, the data will be copied into it, and as a final (and quick) step, there will be a RENAME TABLE to switch out the old table and bring in the new.
With a table that large, you should be careful with the datatypes and indexes. See
http://mysql.rjweb.org/doc.php/myisam2innodb
Also, if you like, provide SHOW CREATE TABLE and some of the SELECTs so we can help you make the resulting table successful.
I assume you have already discovered that the new table is 300-450GB. Either that, or you ran out of disk space, losing the new table, but hopefully leaving the old table intact.