This Q&A was published on the ISAF website today. It answers when rule 18.3 stops applying.
ISAF Racing Rules Question and Answer Service
Published: 9 September 2009
At a windward mark to be left to starboard, Blue is fetching the mark and Yellow tacks in the zone as shown in the diagram. Up to position 2 Yellow breaks no rule and will not do so if she continues to bear away to sail to the next mark. However, Yellow luffs and Blue must sail above close-hauled to continue keeping clear. She does so and protests. Does Yellow break a rule?
Yellow and Blue were approaching the mark on opposite tacks. When Yellow changed tack and as a result was subject to rule 13 in the zone when Blue was fetching the mark, rule 18.3 applied. When rule 18.3 applies, rule 18.2 does not thereafter apply. Blue keeps clear as required by rule 11 and Yellow’s luff complies with rule 16. Yellow causes Blue to sail above close-hauled to avoid her. If either boat is still in the zone, rule 18 continues to apply - see the first sentence of rule 18.1 - and therefore Yellow breaks rule 18.3(a).
This answer will also apply to a 'mirror-image' situation at a port-hand windward mark.
The answer is pretty straightforward. It does however, not mean that Yellow has to bear away to the next mark. There's no rule 17 restriction. Yellow can continue on a close-hauled course until BOTH boats are outside the zone, keeping Blue from sailing down. As long as Yellow does not cause Blue above close-hauled.
As soon as they both are outside Yellow can luff again. This means that if this situation occurs, the umpires have to decide if the luffing took place with one of the boats (most likely Blue) is still in the zone or not.
Rule 18.3 continues to apply, therefore determining when boats have left the zone is now a factor.