Cashier's checks themselves do not expire as personal checks do. However, the funds behind the check are subject to state escheat laws.
A cashier's check is a guarantee from the bank that the funds behind that check are secured and good. There is not an expiration date printed on the front of the check for that reason. However, each state has a specific period of time in which there must be activity on an account or the funds are escheated to the state, which means the bank is required by law to hand that money over to the state. The time period will vary by state, but the most typical time period is five years. If a cashier's check isn't cashed before the time limit, then the bank hands the funds over to the state.