HazCom is the most frequently cited general-industry standard, and training is usually cited for what it did not cover, not for how long ago it happened.
When. Employers must provide employees with effective information and training on hazardous chemicals in their work area at the time of their initial assignment, and whenever a new chemical hazard the employees have not previously been trained about is introduced into their work area (29 CFR 1910.1200(h)(1)). The trigger is a new hazard, not every new product: a second brand of the same solvent class is not new training, a first oxidizer is. Training may be organized by categories of hazard rather than by individual chemical.
What. Employee training must include at least: methods and observations that may be used to detect the presence or release of a hazardous chemical in the work area, such as monitoring, continuous devices, or visual appearance or odor (29 CFR 1910.1200(h)(3)(i)); the physical, health, simple asphyxiation, combustible dust and pyrophoric gas hazards, and hazards not otherwise classified, of the chemicals in the work area ((h)(3)(ii)); the measures employees can take to protect themselves, including the employer's specific procedures, work practices, emergency procedures and PPE ((h)(3)(iii)); and the details of the employer's hazard communication program, including an explanation of shipped-container labels and the workplace labeling system, and the safety data sheet, its order of information and how employees can obtain and use it ((h)(3)(iv)).
What is not there. No annual refresher, no required hours, no required test. "Effective" is the standard, and an inspector tests effectiveness by asking an employee where the SDS binder is and what the pictogram on the drum means.
Employers that train annually anyway are not wrong; they are meeting the state-plan or company rule, not the federal one. The federal duty is triggered by assignment and by change.