It can be tricky to find the expiration date of your membership but if you go here
https://www.delta.com/crownrooms/ann...Landing.action which can be reached by logging in, then selecting "Travel Information", "Airport Information", "Delta SkyClub", "Purchase Membership" it will display your current expiration. Mine now expires in 2015 - having purchased a multi-year membership before DM was announced my residual "credit" has been extended every year after making DM. I also get a new SkyClub card in the mail every year with the new expiry date on it.