Expressing duration in the data privacy vocabulary

Hi,

My name is Alex and I research usable privacy and security for IoT.  I have a question about the actual use of the `duration` attribute of the vocabulary. The specification only says that it exists, in 7.1.26 Storage Duration..

However, what I am wondering about is whether it acts like an "enum" with predefined values such as {none, forever, a week, a month, a year}, or if we are expected to treat it as a "datetime", or some other type related to time-keeping?

Any hints you could provide on this front would be greatly appreciated,
Alex