There should always be a expiry date with it. Just look a little further.

While true that an expiration date should be given, a coupon code that's being sent is always a manual message or e-mail, where one can forget to mention or include the expiration date.

Generally speaking, coupon codes like these given as a replacement for an unused ticket are valid for at least 6 months, although there are exceptions to that rule. If you want to be absolutely sure, the best advice I can give is to contact our Customer Service again and have them check your specific code, if no expiration date has been mentioned.