You could also use a data stream called PresenceData, see the MinimalStatusNotifications sample in the EyeX SDK for C/C++ (equivalent syntax for C#). You will get events whenever the user closes her eyes (long blink) or looks away, and also when the eyes are visible again. If you want to know for how long the user has been non-present you can always save the current time as a variable when you get the non-present event.
EDIT 19 Nov 2014: The UserPresence engine state (formerly known as PresenceData) should not be used for blink detection. The purpose of the state is to tell whether the user is present in front of the screen or not. Even though a current implementation may approximate this with the detection of the user’s eyes, the underlaying implementation may at any time change to not be depending on whether the user’s eyes are open or closed.