Just because a bunch of people in videos are doing illegal things doesn't mean there is a rule (it just means that they haven't been caught yet, or whatever source footage material they are using doesn't use content id or allows use for anyone). In legal terms there is no such rule about 10 seconds of content whatsoever, it's a misinformation myth that people keep spreading and perpetuating thinking it will give them any kind of legal standing (it doesn't). On the topic of parody is a whole different ball park.