Do QR codes ever expire?

No, the QR codes never expire. 

If your QR codes are dynamic, they may become paused after reaching the monthly scan limit of your plan and reactivate in the next month. 

If your QR codes are static, they will neither expire nor get paused. 

To know the difference between static and dynamic QR codes, click on this link: https://www.qrcodechimp.com/what-are-static-and-dynamic-qr-codes-know-the-difference/