Hello Dominique
The specific general-prefix feature is one that functions statically. There are no parameters in which you can configure time sensitive validity of the prefix itself. However, if you do want to achieve this, there are a couple of ways that come to mind.
You can use IPv6 DHCPv6 Prefix Delegation which allows for a DHCPv6 server to provide the general prefix to the customer routers, which in turn provide the specific IPv6 addresses based on those general prefixes. These prefixes can be changed and advertised using DHCPv6 rather than using the static configuration suggested in this lesson.
Alternatively, you can cause a router to change its own configuration internally using EEM scripting. This is a feature that allows you to run scripts or commands when a certain event occurs, or a particular times.
A third possibility is to use device programmability, with tools such as NETCONF, RESTCONF, and gRPC to enable network automation with which you can achieve what you need.
I’m not quite sure if this answers your question, but if not, feel free to clarify!
I hope this has been helpful!
Laz