The short answer is 'it depends'. If there is a history of a certain sample length to define the geology and there is a lot of data then you may be stuck with that. For example a lot of old drilling one deposit I've worked on had 5 foot samples so there was not much choice other than sticking with 2.5 sampling intervals for new drilling. If a new deposit then you need to consider the likely future mining method and selectivity. If large open pit - perhaps two to three metres would be reasonable depending on geological complexity. A good place to look is stock exchange announcements to see what other are doing.

"All QDM drill core is sampled in continuous two metre intervals".

That is a good question. Porphyry Cu deposits are of very substantial size, and thus are typically mined using large-scale bulk tonnage methods. Consequently, ore blocks are usually estimated that are on the order of 5-10 meters in dimension. This means that samples collected from drill core through such deposits produce the best results if they are 1 to 2 m long (I have also seen lots that are 1.5 m long, but I think that is a consequence of geologists who are still working in the English measurement system; 1.5 m is close to 5 feet). I would probably consider 1 m to be the minimum sampling interval for use in a porphyry deposit, and probably 3 m as the maximum (although 1.5 or 2 m is probably the norm and best for most cases). Obviously, if a major lithological boundary occurs within the core, the adjacent samples should be terminated at the contact (and this typically lengthens and shortens the adjacent samples by a little bit). Geostatisticians can easily deal with the statistical issues associated with different sample sizes, so you need not worry about that.