Adding a custom field after post title (ex.Example Title [30 Second Read])

Question

function wpd_title_prefix_filter( $title, $post_id ) {
    if( $checked = get_post_meta( $post_id, '_wpd_title_prefix', true ) ){
        $title = '[30 Second Read]: ' . $title;
    }
    return $title;
}
add_filter( 'the_title', 'wpd_title_prefix_filter', 10, 2 );

$title = ' ' . $title; [30 Second Read]:

Answer 1

You face two challenges in order to achieve your goal.

The first is very simple. If one can be sure that the reading time for every post is 30 seconds, then your function needs an "else" statement to define the reading time as a suffix. The following would work.

function wpd_title_prefix_filter( $title, $post_id ) {
    if( $checked = get_post_meta( $post_id, 'm_meta_description', true ) ){
        $title = '[30 Second Read]: ' . $title;
    }
        else{
         $title = $title.' [30 Second Read]';
        }
    return $title;
}

Your second challenge is more complicated - what if you don't know the reading time. This is well covered in a post titled "How to Display Estimated Post Reading Time in Your WordPress Posts" on wp.beginner.com. It requires a plugin ("Reading Time WP"); the article has full step-by-step details.

Updated at request of OP to change the name of the postmeta key.
I left $post_id as-is because this is a function parameter.