The assignemnt text has been refiend accordingly:
Premium accounts require the payment of a fixed sum coverig a period of time, so enabling an unrestricted access to any content for x time (expressed either in hours or in block height difference), starting from the subscription time.
In general since you are requested to choose x, there should be no need for hour/block height conversion. Just choose one of the two to measure time and stick with it (e.g. if you choose block height, then subscriptions last for x blocks).