It is common for safety consultants and training companies to offer a "Trainer The Trainer" class, video, or certification, but these are not officially recognized or required by OSHA.

For most training subjects, the Trainer just needs to meet certain experience requirements, and be designated by the employer.

Example - Forklift Trainer Requirements

All operator training and evaluation shall be conducted by persons who have the knowledge, training, and experience to train powered industrial truck operators and evaluate their competence. - 1910.178(l)2(iii)

Notice there is no mention of a "Train The Trainer" certification.

Example - Fall Protection

The employer shall assure that each employee has been trained, as necessary, by a competent person qualified in the following areas: 1926.503(a)(2)

In this case, OSHA just says the trainer must be "competent".

Summary

"Train The Trainer" courses and certifications can be a useful tool for teaching an employee to be a trainer, but they are not official designations, and therefore their is no "expiration date" as it relates to OSHA requirements.