When creating a payment (either in or out), you're asked to set an expiry date for that payment (defaulted to 2 days on the Merchant Portal).
What happens after a payment link is expired?
If an end-user opens an expired link, they'll get an error message like the one below:
However, if the end-user saves the payment address before the expiry time, they can still proceed with the payment. Check out "What should I do in case of a late payment?"