The length of time for which the training is valid is determined by OSHA 29 CFR § 1910.132 Personal Protective Equipment:
The employer shall provide training to each employee who is required by this section to use PPE.
When the employer has reason to believe that any affected employee who has already been trained does not have the understanding and skill required by paragraph (f)(2) of this section, the employer shall retrain each such employee. Circumstances where retraining is required include, but are not limited to, situations where:
Changes in the workplace render previous training obsolete; or
Changes in the types of PPE to be used render previous training obsolete; or
Inadequacies in an affected employee’s knowledge or use of assigned PPE indicate that the employee has not retained the requisite understanding or skill.