Guest requests, both authoring and audio, are not time sensitive and do not expire based upon a time or date.
However, you may only have one active link at a time, per presentation. As soon as you generate a new request for a presentation, any other request previously generated for that presentation will expire.
Also, the quest links are good for one use only. They expire after they are used. If an author uses a link to record audio, a new link must be generated if another author is going to record audio or if the audio needs to be rerecorded.