The WeChat group QR Code is valid for 7 days (starting from the time it is generated; the expiration time is shown below the QR Code). Once expired, scanning the code will prompt a message saying this QR Code has expired.
If you leave the group after sharing its QR Code, the code will immediately become invalid. If all members have left the group, scanning the QR Code will display a message indicating that the group no longer exists.
Tap here to learn how to use the group QR Code.