* Transferability: The acceptability of units earned at one institution to another depends entirely on the receiving institution's policies. Some colleges have time limits on how old transfer credits can be. This limit varies wildly from school to school, and sometimes even from program to program within a school. You might find some institutions accepting credits up to 10 years old, while others might only accept credits from the last 5 years, or even less. You'll need to check the transfer credit policies of the specific school you intend to transfer to.
* Program Requirements: A program itself might have requirements stating that credits must be earned within a certain timeframe to count toward graduation. This is common, particularly in rapidly evolving fields. Check the specific program requirements at your target school or program.
* Degree Completion: There's typically a time limit for completing a degree program. This can range from 5 to 7 years (and sometimes longer), and if you don't complete the degree within that timeframe, you may need to reapply or retake some courses. This is not so much about the units themselves expiring, but the overall program's time limitations.
In short, while the units themselves don't expire, their *acceptability* for transfer or use toward a specific degree can be limited by the policies of the receiving institution or the program requirements. Always contact the admissions or registrar's office of the college or university you plan to attend to determine their specific policies.