Audio messages, introduced in iOS 8, allow you to send short voice clips via iMessage. They're incredibly easy to use and receive which should make them a popular option when folks are unable to text. However, they can take up far more storage space than regular text-based messages. That's why you're able to edit automatic expiration settings for audio messages to suit your needs.
How to change expiration settings for audio messages on iPhone and iPad
Launch the Settings app on your iPhone or iPad running iOS 8 or higher.
Tap on Messages.
Tap on Expire under the Audio Messages section.
Choose either Never or After 2 Minutes.
That's it! If you frequently find yourself scrounging for storage space on your iPhone or iPad, it's probably a good idea to set them to expire after 2 minutes. Don't worry, that means 2 minutes after you listen to them, not 2 minutes after you receive them.
It'd be nice if Apple would add an in-between option but until then, you'll need to make a choice. So for now, what are you selecting? Let me know in the comments!