Cache form Drupal change expire time
Look at the function that's used to save form "cache", form_set_cache:

function form_set_cache($form_build_id, $form, $form_state) {
  // 6 hours cache life time for forms should be plenty.
  $expire = 21600;


As you can see, it's hardcoded and you can't really change it without hacking core. Now the question is - should you?

{form_cache} as cache

Form data is kept as a cache bin, but it is not a "real" cache. Let's look at the definition from Wikipedia:


  In computer science, a cache (/ˈkæʃ/ KASH) is a component that transparently stores data so that future requests for that data can be served faster.


Now, that's pretty much what {form_cache} is not - it is kept to save temporary data that needs to be consistent, and that are never stored anywhere else during form processing. It means that when someone is using APC or Memcache, he needs to take extra care to prevent their "it can be deleted without any reason, for performance purposes" approach from corrupting forms. Most Drupal admins I know simply exclude form cache bin from them.

If you will shrink the time data is kept, you risk all kinds of errors. So keep in mind that your form cache bin should preserve data significantly longer than you expect anyone to need between first form display and the end of it's processing.