How much space do audio recordings made by the stock audio recorder on iOS use per minute?
The amount of space any recording takes will depend on a number of factors. And, even the exact same recording can take up different amounts of space depending on the drive it's stored on and how it was formatted.

In my experience, a one minute audio voice recording using the Voice Memos app on an iPhone 6s with iOS 11 installed will typically take between 650KB and 700KB. That is based on one person speaking into the phone while they're in a quiet room. Note that the amount of background noise, distance from the microphone, etc can all affect the amount of space required.

The Voice Memos app saves audio as a MPEG 4 Audio file (i.e. a file with the .m4a extension).