Answer:
A prescription for Acyclovir or any prescription drug for that matter, is good until the expiration date on the manufacture bottle that the pills originally came from. The expiration dates are usually 3-5 years from date that the pills were made. So from the time that you pick your pills up from the pharmacy you have approximately 2-3 years on average. And even after the pills expire, they aren't going to cause you any harm or anything. It's just that the pills lose their effectiveness.