Quite a hot topic. It depends first on local regulations, some of them allow quite long flight duty time without the need to augment the crew (we do up to 13 hours currently). And then of course different regulations and different carriers use different rules to discern if a crew has to be augmented.
Although usually regulations allow a maximum time to be flown, some carriers use nautical miles as the measure when to use what kind of crew supplement. Nautical miles, if chosen low enough, has the charm that different cost indices cannot descide about crew augmentation or not. However it is really a carrier by carrier thing.