I believe Kurt was up at seven years. I'm at five.
Based on the accelerated aging I've seen at room temperature, I do not think nicotine acts like milk, where it's good for a period of time and then suddenly goes bad. In fact, its degradation appears fairly linear to me, and based on the fact that I've seen essentially none in 5 year old nic that's been properly stored, I believe we can count on it to stay usable for decades.
Nic Base Storage: Freezer vs. Room Temp Experiment
When your other choice is nothing, seems quite a realistic approach.
I have 5 liters in the freezer and have been mixing and freezing eliquid with pre made nic base so I don't touch the nic stash. Still have enough nic base to freeze quite a bit of eliquid. Just have to order more bottles.