I can't find any mention of this on any official page. Actually, there is hardly any publicly available information about the NSC. Did you ask whoever passed on this rumour what their source was?
I doubt that it is true there is definite validity for any length of time, because I would expect the authorities to reserve the right to check a person out again in the light of any new activities they might hear about - for instance, if perhaps the person made a trip to one of the countries that is a reason for NSC in the first place. (This would be the same as for medicals, where the same medical can be used for another application normally, but INZ can require another if they think the person's health might have been put at risk.)