How to increase default Drupal minimum cache lifetime options to a custom value?
Just set the variable 'page_cache_maximum_age' using variable_set in custom module to the value 31536000

variable_set('page_cache_maximum_age', 31536000);


If you install the Devel Module there is a page for viewing and editing variables.

Above is a quick fix. If you want to do a perfect way then you need to form alter the "system_performance" form and change the option of below field.

  $period = drupal_map_assoc(array(0, 60, 180, 300, 600, 900, 1800, 2700, 3600, 10800, 21600, 32400, 43200, 86400), 'format_interval');
  $period[0] = '&lt;' . t('none') . '&gt;';
  $form['caching']['cache_lifetime'] = array(
    '#type' =&gt; 'select',
    '#title' =&gt; t('Minimum cache lifetime'),
    '#default_value' =&gt; variable_get('cache_lifetime', 0),
    '#options' =&gt; $period,
    '#description' =&gt; t('Cached pages will not be re-created until at least this much time has elapsed.')
  );
  $form['caching']['page_cache_maximum_age'] = array(
    '#type' =&gt; 'select',
    '#title' =&gt; t('Expiration of cached pages'),
    '#default_value' =&gt; variable_get('page_cache_maximum_age', 0),
    '#options' =&gt; $period,
    '#description' =&gt; t('The maximum time an external cache can use an old version of a page.')
  );