Hey Kate - see my reply on your other post!
Hi Kate,
You can't use time-based criteria for Program completion directly, but you could probably arrange something analogous using the Minimum score functionality where you could create a notional time value to each course.
The minimum score inline help under the program content tab states:
A minimum score that must be reached for the course set to be marked as complete. The score is calculated by summing the values from the Course score field for all the courses a user has completed. For example, you might have a Credits custom field on your courses, so you could use this setting to mark a course set as complete when a certain number of credits have been achieved.
Only whole numerical values should be used in this field.
Hope this is useful,
cheers,
George.