Final answer:
An Engineer Technical Letter can remain in effect indefinitely or have a specific expiration date depending on the organization that issued it.
Step-by-step explanation:
An Engineer Technical Letter is a document that shares technical information related to engineering. The length of time an Engineer Technical Letter remains in effect can vary depending on the specific letter and the organization that issued it. In some cases, an Engineer Technical Letter may be valid indefinitely, while in other cases it may have a specific expiration date or be revised periodically. For example, an Engineer Technical Letter related to a specific code or standard may remain in effect until that code or standard is updated or revised.