I'd try to do it at least once a year. Use nothing less than the Honda Type 2 Coolant.
With that said, advances in the coolant since the NSX came out might be ok to run it longer than the standard 2/3 year manual interval....
You can always do a coolant test to see if it's too early or late. This is a significant maintenance item - most overheating issues are because of coolant, the AF + chemicals lubricates the water pump, minimizes corrosion etc...