I would say it is good practice to change the gearbox oil & diff oil at least every 30-50K miles under normal conditions ~ not necessarily a necessity but just good practice. Don't even think it's listed in the service schedule for the 350Z.
How long the oil will last is anyone's guess as that will depend on working conditions, temperatures, water contamination (more so in humid climates), general use (hard or not), wear contaminants (like metal particles from gears) & more.
Changing the gear oils in either the diff or gearbox is a lot cheaper than replacing the mechanical parts inside so it makes good sense to me to get it done on any new vehicle I own. My 2 pence worth.