A 70m rope (depending on the building) should manage 7 floors with a double strand (only just, and you'd want to use some webbing to anchor to the edge). The fact that he mentions a tibloc indicates he was rapping on a single strand though as that device would not handle a double strand ascension. I doubt I'd be comfortable anchoring to an AC unit though. Also, really, don't try this at home. There are lots of non-obvious hazards here. For instance, climbing ropes are incredibly strong and in practice, just don't break, unless, oh, say, they are weighted over a sharp edge like the corner of a building. And no, do not try the "shake the (insecure) knot free" technique, really. Rappelling is (statistically) very dangerous as it is one of the few situations in climbing where you are trusting your life to a single point of failure (redundancy is good! and is probably responsible for more deaths than climbing falls are [1]

[1]: http://publications.americanalpineclub.org/search/solr?all=R...