There isn't a direct way to tell when it expires via the software in version 2.7. Version 3.0 has the ability to tell when it expires if you are interested in upgrading. You will have to consult the email you received when you purchased the license. It will tell you the expiration date. If you cannot find the email, you can request you license information again by going here: http://www.eset.com/support/lost_license.php and entering your information. HTH
as suggested - use the lost password feature - but if you don't have access to the email address used to purchase the email (because the email is sent to the address on record), call your reseller or Eset directly.