Get custom fields values in filter on wp_insert_post_data

Tweak to Riadh's accepted answer (would add as a comment but haven't got enough rep yet):

As documented in the WordPress Codex wp_update_post includes the save_post hook so calling wp_update_post() inside the save_post hook creates an infinite loop. To avoid this, unhook then rehook your function like so:

add_action('save_post', 'change_title');

function change_title($post_id) {
    $time = get_field('time',$post_id);
    $post_title = 'Topic created at '. $time;

    // unhook this function so it doesn't loop infinitely
    remove_action('save_post', 'change_title');

    // update the post, which calls save_post again
    wp_update_post(array('ID' => $post_id, 'post_title' => $post_title));

    // re-hook this function
    add_action('save_post', 'change_title');
}    

You can actually access the global $_POST variable for your field value , but i guess you can do it in a cleaner way by using the save_post action to update your post's title, eg:

add_action('save_post', 'change_title');

function change_title($post_id) {
    $time = get_field('time',$post_id);
    $post_title = 'Topic created at '. $time;

    // unhook this function so it doesn't loop infinitely
    remove_action('save_post', 'change_title');

    // update the post, which calls save_post again
    wp_update_post(array('ID' => $post_id, 'post_title' => $post_title));

    // re-hook this function
    add_action('save_post', 'change_title');
}  

assuming that your ACF fieldname is "time".

Edit: Updated the answer as per Mark Chitty's answer.