An activity generally only expires when you tell it to, e.g. via an escalation that makes an activity "time out" after a certain period of time.

However, an activity instance destination can also "expire" when a task is completed.  For example:

• An activity is assigned to a K2 role or AD group.  When the activity is completed, the destination for the group/role will be expired but an activity instance destination will be created for the group member who actually completed the task with the detials of the action.
• An activity is assigned to users A and B.  The activity may be completed by either user.  If user A completes the activity, the desintation for user B will be expired and vice versa.
• An activity is assigned to five users.  The activity must be approved by 3 of the 5.  Once three users have approved, the destinations for the remaining users (if any) will be expired.

There are probably some other scenarios, but these are the most common ones.