How long can a brand new PCB last before additional protective coating is required if any?
It will last as long as you keep condensation away from it or other solvents. Ionic solutions are what degrade metals, so the easiest way would be to keep it from those things. 

That being said, feel free to apply your own soldermask

Polyurethane (humiiseal) also may go a long way toward keeping moisture out. 


Source: https://learn.adafruit.com/how-to-solder-mask-pcbs/overview

Make sure you either, mask the metal you still want to solder to before applying the soldermask. And conformal coatings will coat everything and make for hard reworking and should be applied after assembly.