"Players are entitled to an interval at half-time, not exceeding 15 minutes; a short drinks break (which should not exceed one minute) is permitted at the interval of half-time in extra time. Competition rules must state the duration of the half-time interval and it may be altered only with the referee’s permission."
My interpretation is it is 15 minutes unless the referee allows something else.
That's a stretched interpretation. The first sentence says the interval does not exceed 15 minutes. The most natural way to read the last sentence is that the competition rules state how long the half-time is under those parameters (i.e., the rules can state that the interval is 10 minutes, but not 1 hour); likewise, the referee can allow the competition organizers to change the duration of the interval -- the competition rules notwithstanding --, but still within the parameters of the 1st sentence.
My interpretation is it is 15 minutes unless the referee allows something else.