How long is change set associated with work item?

Question

This change set is not delivered to the stream.
I can view the change set files by viewing the work item. If the repository workspace is deleted will the change set become dissociated with the work item ?

No, the association remains.

From this question:

the change set isn't deleted. It will still exist in the repository but may be difficult to find.
If you haven't associated a work item with the change set and it doesn't exist in another repository workspace or stream, you would have to find it by searching for change sets and specifying properties like the owner, creation date, files changed, etc.

It's not that the files are removed from the source component. It's the configuration of that component (ie. the change sets) that is lost because no backup baselines are created for the components before deleting the repository workspace.

What are the circumstances in which a change set becomes dissociated with a work item ?

You can explicitly remove a change set from a work item through the same menu (Associate with Artifact > Work Item) you are using to associate a change set to said work item in the first place.
And that makes sense, since you can associate it to several Work Items.