if(strtomtime('now')>get_the_author_meta('expire_by', $user_id)){
// let the user post
} else {
// prevent from posting
// show an error
}

add_action( 'transition_post_status', 'check_plan_validity', 11, 3 );
 function check_plan_validity( $new_status, $old_status, $post ) {
     $expire_by = (int) get_the_author_meta('expire_by', $post->post_author);
     $now = strtotime("now");
     if($now>$expire_by){
         $error['error'] = 'Plan expired';
         echo json_encode($error);
         die();
     }
 }