I have just looked at my ACRO which I got earlier this year. There is only the date of issue printed on it. There is no expiry date as you would expect - it was accurate at the date of issue.
There could be a point some time in the future when you might have had time to carry out some crimes in the UK, and so might need a new one! However mine was accepted 6 months after it was issued so in the short term I wouldn't worry.