// Save Custom Field
add_action( 'woocommerce_process_product_meta', 'woo_save_custom_shipping_fields' );

function woo_save_custom_shipping_fields( $post_id ){

    $woocommerce_text_field = $_POST['delivery_time_frame'];
    if( !empty( $woocommerce_text_field ) )
        update_post_meta( $post_id, 'delivery_time_frame', esc_attr( $woocommerce_text_field ) );

    if( empty($woocommerce_text_field)):
            delete_post_meta($post_id,'delivery_time_frame');
    endif;

}