Does tea lose its potency over time? How should it be stored to keep it fresh?

It's depends on the type of tea you have. But, basically, keep it in the dark place in the metal box or glass bottle with cover. The most important is good cover to close the tea and save the aroma. And don't keep it for long time and check the expiry date.

Yes. Tea can and will lose its potency/flavor over time. It should be stored in a air tight container and away from light.