Example Drupal 7 Utility Module

cache_form_util.module

<?php

/**
 * @file
 * Provides cache utility functions
 */

/**
 * Implements hook_menu().
 */
function cache_form_util_menu() {
  $items['admin/cache_form_util'] = array(
    'title'             => t('Cache Form Utility'),
    'page callback'     => 'drupal_get_form',
    'page arguments'    => array('cache_form_util_callback'),
    'access callback'   => 'user_access',
    'access arguments'  => array('administer cache form util'),
    'type' => MENU_NORMAL_ITEM,
  );

  return $items;
}

/**
 * Admin cron config form
 */
function cache_form_util_callback($form, &$form_state) {
  $form['cache_form_util'] = array(
    '#type'         => 'fieldset',
    '#title'        => t('Cache Form Utility'),
    '#collapsible'  => TRUE,
    '#collapsed'    => FALSE
  );

  $form['cache_form_util']['cache_form_util_cache_form_clear_limit'] = array(
    '#type'           => 'textfield',
    '#title'          => t('Limit for cache_form pruning'),
    '#default_value'  => variable_get('cache_form_util_cache_form_clear_limit', 1000),
    '#description'    => t('Make cron limit the number of expired entries on cache_form')
  );
  $form['cache_form_util']['cache_form_util_cache_form_clear_interval'] = array(
    '#type'           => 'textfield',
    '#title'          => t('Cron Job Time Interval'),
    '#default_value'  => variable_get('cache_form_util_cache_form_clear_interval', 60),
    '#description'    => t('Time in minutes between cron job excution times.')
  );

  return system_settings_form($form);
}

/**
* Impliments hook_cron().
*
* Remove a limited number of objects from the cache_form table.
*
*/
function cache_form_util_cron() {
  $limit = variable_get('cache_form_util_cache_form_clear_limit', 1000);

  // Default to an hourly interval. Of course, cron has to be running at least
  // hourly for this to work.
  $interval = variable_get('cache_form_util_cache_form_clear_interval', 60) * 60;

  // We usually don't want to act every time cron runs (which could be every
  // minute) so keep a time for the next run in a variable.
  if (time() >= variable_get('cache_form_util_cache_form_clear_next_execution', 0)) {
    watchdog("Cache Form Util", t('Selecting up to !limit removable items...', array('!limit' => $limit)));

    // Get $limit number of cached forms that are older than the expire default.
    // By default, expiration is 6 hours after the time an entry was written
    $results = db_select('cache_form')
      ->fields('cache_form', array('cid'))
      ->condition('expire', CACHE_PERMANENT, '<>')
      ->condition('expire', REQUEST_TIME, '<')
      ->range(0, $limit)
      ->execute();

    $cids = $results->fetchCol('cid');

    if (empty($cids)) {
      return 0;
    }

    watchdog("Cache Form Util", t('Deleting !count items...', array('!count' => count($cids))));

    // Delete the batch of expired entries.
    $deleted = db_delete('cache_form')
      ->condition('cid', $cids, 'IN')
      ->execute();

    watchdog("Cache Form Util", t('!count items deleted', array('!count' => $deleted)));
    variable_set('cache_form_util_cache_form_clear_next_execution', time() + $interval);
  }
}