You need to record, in the database, when the email was sent and then compare that date with the date that the user clicks the link. So, for example, you send the email at 31/12/2012 18:22 and I click on the link on 01/01/2013 18:22 then that is only 1 day.
You will need to build logic in to either a stored procedure or your asp.net code that runs on the confirmation page that checks that DateSent field
Edit
You 'expire' the link by making it valid only before a specific date. If the user tries to use the link after the date then you treat it as expired and show a relevant message. But you need to store the date in the database, in my original suggestion I've said to store the date you send out the email (in a DateSent field) and compare against that, another answer suggests setting the date the link expires (in an ExpiryDate field). Either approach will work, they are just coming at the problem from different ends of the problem.