A Pay Now link, once generated, will last 30 days. If the link isn't visited within that period, the link will expire and no longer be valid.
It is important to note that this 30 day expiration is for a link that has not yet been visited. Once a link is visited by a customer, a secure page is generated by the gateway, which will expire within a relatively short amount of time.
This means that once the link is visited, if the customer fails to input their credit card information before the gateway page expires (which usually happens after 5-10 minutes), the import into iPayment will fail, and the customer will need a new Pay Now link.