The number (90 mins) actually comes from CS25 ... and now we really are getting a bit dodgy so far as the syllabus is concerned ... but we can cope ...

They are talking about ... flights across land areas which have been designated by the State concerned as areas in which search and rescue would be especially difficult, they must be equipped with at least one ELT(s), pyrotechnic signaling devices and additional survival equipment taking into account the number of persons on board, except that the additional survival equipment need not be carried when the aeroplane either:

.... for aeroplanes certificated to JAR-25 or equivalent, no greater distance than that corresponding to 90 minutes at cruising speed from an area suitable for making an emergency landing.