How does an alter work?
An ALTER TABLE operation that adds a column to a table should be a simple data dictionary only update.  There is no reason for Oracle to visit all the blocks/rows in the table.  So, the operation should be very fast.  (sub-second response time.)

The only exception to this would be, if you're on a version of Oracle prior to 11g, and you're setting a default value for the column being added, Oracle has to visit all the rows.  In 11g, even setting a default value will not precipitate a visit to all the rows in the table.