if ( isset( $_POST['slide_duration'] ) && is_numeric( $_POST['slide_duration'] ) && $_POST['slide_duration']  >= 1 ) {
    $slide_duration = intval($_POST['slide_duration']);
    update_post_meta( $post_id, 'slide_duration', $slide_duration );
}
else {
    delete_post_meta( $post_id, 'slide_duration' );
}

if ( isset( $_POST['background-video'] ) ) {
            update_post_meta( $post_id, 'slide_duration', $video_meta['length'] );
        }

<?php
    $slide_duration = isset( $theme_stored_meta['slide_duration'] ) ? intval($theme_stored_meta['slide_duration'][0]) : '';
?>
<input type="number" name="slide_duration" value="<?=$slide_duration?>" min="1" step="1" style="width: 70px;" /> <?=__('seconds', 'theme')?>