WeatherFlow Support’s answer to this question via my support ticket:
Tempest data is stored in the cloud indefinitely and is always accessible. As of now, device data history is kept forever. Only internal calibration bias corrections are periodically cleansed after several months of storage. I am not aware of any limitations beyond only being able to access data from your own network of Tempest stations unless you are subscribed to a higher tier allowing access to a wider weather station network accessible through the API.