Taken right from the delta website...
Account Deletion and Mileage Expiration
Accounts with no activity for 12 consecutive months after enrollment will be deleted. A new SkyMiles number can be obtained by contacting Delta.
Currently, miles will not expire as long as you participate in one of the following activities
at least once every two years:
- Earn miles for travel on a qualifying Delta, Delta Shuttle®, SkyTeam, or other SkyMiles airline partner flights.
- Earn or redeem miles with one of the SkyMiles program partners including hotels, car rentals, Delta SkyMiles Credit Card from American Express, international credit card partners, SkyMiles Dining & Hotels by Rewards NetworkSM, MilePoint.com, mortgage lenders, CAP magazine subscriptions, real estate, or telecommunications partners.
- Buy Miles here at delta.com or by calling the number on the back of your SkyMiles card.
- Redeem miles on Delta, Delta Shuttle, a SkyTeam partner, or another SkyMiles airline partner. Redemption of previous program awards does not extend the mileage expiration date.