DEVELOPER.txt in Autocomplete Limit Length 7

Developers can specify a per form-element minimum autocomplete length by
setting the data-limit attribute on the element in question. If found,
the module will use this value rather than the global value set via the
admin page.

Here is a simple example for a standard textfield element.

/**
 * Implements hook_form_alter().
 */
function example_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'test-form') {
    $form['element']['#attributes']['data-limit'] = 7;
  }
}


Note that not all autocomplete fields have the same form element structure.
For example, taxonomy term reference fields use a sub-element:

/**
 * Implements hook_form_alter().
 */
function example_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'test-form') {
    $form['element'][LANGUAGE_NONE]['#attributes']['data-limit'] = 3;
  }
}