When does the exemption expire?

In most states, the exemption does not expire nor need to be renewed. However, there are situations which occur that the sales tax is put back on the bill, and/or a new utility study must be completed.
Expiration states: Texas – 5 yrs;  Michigan – 4 yrs;  Iowa – 3 yrs;  PA – 5 yrs ; Vermont – 3 yrs. ; Wyoming – 3 yrs.; CT – 6 yrs