When is the most effective time to provide workers with Hearing Conservation Training?

It's best to provide workers with Hearing Conservation training at the time of their annual audiogram.

Answer Extra: 

In a report by the UK Health and Safety Executive (HSE), at worksites where managers reportedly provided training in the use of hearing protectors, about two-thirds of the workers could not recall or recognize having been trained. Only 20% of workers recalled receiving training on factors that limit the effectiveness of hearing protectors, even though all their managers reported providing that training.

There is one standout aspect of hearing conservation that was very memorable for workers: the audiometric test. Fully 83% of workers at companies administering hearing tests reported receiving the test. Many employers find that the most effective time for administering hearing conservation training is in conjunction with the annual audiometric testing, when the worker is tuned into his personal safety and health.

source: 
G.W. Hughson, R.E. Mulholland, H.A. Cowie, “Behavioural Studies of People's Attitudes to Wearing Hearing Protection and How These Might Be Changed.” Research report 028 of the Institute of Occupational Medicine (2002), Edinburgh, UK.