No requirements exist that I am aware of. The standards Docs were purely advisory and, in many cases, a legacy of pre-JAA CAA standards that they were trying to apply to later JAA regulations. Eventually the CAA agreed that they could and should only insist on compliance with JAA/EAS regulations and so the advisory docs have slipped even further into the background.
How much teaching a TKI does should depend on a number of factors, including experience and the type of teaching. For continuous teaching 18 hours is about right for a new-ish TKI although many ATOs work them harder. In short bursts instructors can easily cope with more than 24 hours a week, in fact contract instructors would find their income restricted if that rule was applied. We have periods where there is no teaching, and periods where the teaching can be full on for a couple of weeks. Ultimately it should be down to the ATO and its employees/contractors to establish a working environment that is efficient and comfortable otherwise you just won't hang on to good employees. Good TKIs are hard to come by.