I think they are good for around 60k to 70k from new. You can probably skip this time. I might be wrong (someone please correct me) - I think your zed will go into limp mode if any plug failed and dash lights, don't think it will destroy your engine (unless you continue to drive it).
Have a look at the guide here. 6 x plugs will be around £60-£70 hence £100 for labour (around 2 hrs max for a trained mechanic). If I'm doing it, I'm expecting around 4 hrs. I find joy in disassembling and assembling stuff......I'll learn more about my zed too. You'll probably do a better job than a Nissan mechanic as they are most likely to be as clueless as you are, they don't get many 350z going in.