<?php global $bp, $forum_template; ?>

<a href="<?php echo $bp->root_domain . '/' . BP_GROUPS_SLUG . '/' . $forum_template->topic->object_slug . '/forum/topic/' . $forum_template->topic->topic_slug . '/#post-'. $forum_template->topic->topic_last_post_id; ?>"><?php bp_the_topic_time_since_last_post() ?> ago</a>