Changed set_cookie() function to use new cookie_expiration option.

add_form_options.php

<!-- Add this section after the single entry table row (Line 70) -->
<tr id="cookie_expiration">
    <td style="padding-left:75px;" valign="top"><label><?php _e('Cookie Expiration', 'formidable') ?></label></td>
    <td>
    	<input type="radio" name="options[cookie_expiration]" value="midnight" id="cookie_expiration_midnight" <?php checked($values['cookie_expiration'], 'midnight') ?>/> <label for="cookie_expiration_midnight"><?php _e('Midnight', 'formidable') ?></label><br/>
    	<input type="radio" name="options[cookie_expiration]" value="one-day-from-now" id="cookie_expiration_one_day" <?php checked($values['cookie_expiration'], 'one-day-from-now') ?>/> <label for="cookie_expiration_one_day"><?php _e('One day from now', 'formidable') ?></label><br/>
    	<input type="radio" name="options[cookie_expiration]" value="one-week-from-now" id="cookie_expiration_one_week" <?php checked($values['cookie_expiration'], 'one-week-from-now') ?>/> <label for="cookie_expiration_one_week"><?php _e('One week from now', 'formidable') ?></label><br/>
    </td>
</tr>

<!-- Add this to the script block at the bottom of the page -->
jQuery(document).ready(function() {
	jQuery('#single_entry_type').change(function() {
		if (jQuery('#single_entry_type option:selected').val() == "cookie") {
			jQuery('tr#cookie_expiration').css('display','block');
		} else {
			 jQuery('tr#cookie_expiration').css('display','none');
		}
	});
});

FrmProEntriesController.php

/* AJAX */
    function set_cookie($entry_id, $form_id){
    	global $frm_form;
    	$form = $frm_form->getOne($form_id);
    	$form->options = stripslashes_deep(maybe_unserialize($form->options));
    	$expires = time() + 30000000;
    	if ( $form->options['cookie_expiration'] == 'midnight' ) {
    		$expires = mktime( 23, 59, 59, date( 'm' ), date( 'd' ), date( 'y' ) );
    	} elseif ( $form->options['cookie_expiration'] == 'one-day-from-now' ) {
    		$expires = time() + 60 * 60 * 24 * 1;
    	} elseif ( $form->options['cookie_expiration'] == 'one-week-from-now' ) {
    		$expires = time() + 60 * 60 * 24 * 7;
    	}
        //setcookie('frm_form'.$form_id.'_' . COOKIEHASH, current_time('mysql', 1), time() + 30000000, COOKIEPATH, COOKIE_DOMAIN);
        setcookie('frm_form'.$form_id.'_' . COOKIEHASH, current_time('mysql', 1), $expires, COOKIEPATH, COOKIE_DOMAIN);
        die();
    }

FrmProFormsHelper.php

// Added 'cookie_expiration' into the list of default options
function get_default_opts(){
        global $frmpro_settings;
        
        return array(
            'edit_value' => $frmpro_settings->update_value, 'edit_msg' => $frmpro_settings->edit_msg, 
            'logged_in' => 0, 'logged_in_role' => '', 'editable' => 0, 
            'editable_role' => '', 'open_editable' => 0, 'open_editable_role' => '', 
            'copy' => 0, 'single_entry' => 0, 'single_entry_type' => 'user', 'cookie_expiration' => '',
            'success_page_id' => '', 'success_url' => '', 'ajax_submit' => 0, 
            'create_post' => 0,
            'post_type' => 'post', 'post_category' => array(), 'post_content' => '', 
            'post_excerpt' => '', 'post_title' => '', 'post_name' => '', 'post_date' => '',
            'post_status' => '', 'post_custom_fields' => array(), 'post_password' => '',
            'plain_text' => 0, 'also_email_to' => array(), 'update_email' => 0,
            'email_subject' => '', 'email_message' => '[default-message]', 
            'inc_user_info' => 1, 'auto_responder' => 0, 'ar_plain_text' => 0, 
            'ar_email_to' => '', 'ar_reply_to' => get_option('admin_email'), 
            'ar_reply_to_name' => get_option('blogname'), 'ar_email_subject' => '', 
            'ar_email_message' => '', 'ar_update_email' => 0,
        );
    }