Form timing out with drupal_set_message
There's an infinite loop in your code...

The function zillow_api_info() returns a call to itself, which will result in an infinite recursion from which PHP can't escape. As you're calling this function during the call to drupal_set_message() I'd guess that's why you're getting the timeout.

FYI You're not passing the @zipcode replacement to the t() function, it shouldn't cause an error but it might be a bug :)