Check effect of hook_schema_alter() hook
This hook is not used to edit the database as such. It is used to alter the schema when using the drupal_get_schema


  Gets the schema definition of a table, or the whole database schema.
  
  The returned schema will include any modifications made by any module
  that implements hook_schema_alter().


To do what you want you will have to use the db_change_field and this would go in the .install file. 

You can check out the Schema API to get other relavent functions

You have to write db_change_field() in hook_enable() function.

Here is my modified answer:

/**
 * Implements hook_enable().
 */
function comment_alter_enable() {
    db_change_field('comment', 'subject', 'subject', array(
        'type' =&gt; 'varchar',
        'length' =&gt; 100,
        'not null' =&gt; TRUE,
        'default' =&gt; '',
        'description' =&gt; 'The comment title.',
        )
    );
}