#CACHE{duration} is used to define the duration that the cache will be valid for after calculation of a template, expressed as a number of seconds. When this duration is exceeded, the template will be freshly calculated the next time it is requested.
This tag is generally inserted at the top of template files. In its absence, by default, the validity duration of a page is for 24 hours (defined by the constant _DUREE_CACHE_DEFAUT).