Each Authentication Code is valid for 5 minutes, unless a new code has been sent to you. You will get an error if you enter a wrong code, or it has been more than 5 minutes since the code was sent to you or you are using an old code.
You might be interested in the following questions
How do I know that I have a payment order pending for my approval?
Why do I need an Authentication Code or Approver Code to approve/reject a payment order?
Can I approve a payment order without an Authentication Code or Approver Code?
What is the difference between Authentication Code and Approver Code?
How can I get an Authentication Code to approve or reject a payment order?
How can I get an Approver Code to approve or reject a payment order?
How can I approve orders if I am unable to verify my mobile number?
I received multiple Authentication Codes, which one should I use to approve or reject an order?
I am getting an error that my Authentication Code is not valid or has expired?