i.e. do they last forever or will they expire after some amount of time?
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I assume you are referring to the link shared with the end user using the Send & Track feature?
The link doesn't have any expiration limit or you can say a time limit of stop being working. As long as you do not delete it manually or revoke it from your end, it stays there at the end user's end.
If any point of time you would like to delete it, then you can follow the steps suggested here- Disable a "send & track document link" once its been created?
Hope this helps.