Here's my current thoughts (drawing from other people's ideas, not all originally my own ideas):
IMHO the morally defensible copyright length would be no more than what is democratically agreed, benefits the public good, and doesn't infringe on personal liberty. If copyright were to cover commercial use only, then I think personal liberty is largely covered. Research suggests 15 years is economically optimal (I think this is WRT commercial works, the reference is on the wiki policy page), so IMHO a democratically agreed term up to 15 years, but no more, is morally defensible.
In the case of software, RMS has raised a concern that, if we are to keep copyright at all, a reduction to a copyright term of around 5 years could harm F/OSS by tipping the playing field in the favour of commercial software (in the article How the Swedish Pirate Party Platform Backfires on Free Software
linked). Commercial software could build upon F/OSS once the copyright expired, but F/OSS could still not build upon closed source commercial software. This may assist commercial software to draw away users, and therefore developers, from F/OSS, and discourage contributions to F/OSS in the first place. I think this may be a valid concern. RMS suggested a reduction to a 10 year term probably wouldn't tip the playing field too much. (He also suggested other options, which, although interesting, don't have much support in the pirate movement AFAIK, and may be too confusing for promotion to the general public at this stage).
In regards to political expediency, it is a major concern to me that being too ambitious could limit our chances of success. I personally suspect that, in the current climate, anything less than 10 years would seriously risk political suicide by casting us as 'extremists'. I think it is fair to say that realistically we are not likely to achieve even this in the near future, and I think a 15 year term may be significantly easier to promote in sound bites and debates. In arguing for a 15 year term, we can point to research that suggests this is economically optimal, and an opposing argument must then challenge the research, or argue for being economically suboptimal. OTOH, we have already promoted a 10 year term, and this would be a late switch.
I think international cooperation would be very beneficial, since we are up against international treaties and agreements, so it would be good to be consistent with the international pirate movement as much as possible to present a united front. Terms of 5, 10, 15, and 20 years are specifically mentioned by other pirate parties, some pirate parties simply suggest reducing copyright term without being specific, and some appear not to address reducing copyright term at all, but rather only applying copyright only to commercial use. The Uppsala Declaration states "expires well within one generation". I think a generation is traditionally taken to be 20 years, so I think a 20 year copyright term would technically not even be within
a generation, let alone well
within. A term of 15 years is 25% less than 20 years, and I think this can reasonably be considered "well within one generation", so I think 5, 10, and 15 years are all consistent with the international pirate movement.
On this basis, I think it would be best to set PPNZ policy for copyright term at either 10 or 15 years, and I am currently leaning towards 15.