As the work health and safety laws do not define 'frequently', the ordinary meaning applies. The frequent use of hearing protection means using it often or commonly. In other words, if a worker is required to wear hearing protection as part of their normal workplace activity, that would be considered ‘frequently’.
However, if a one-off or unusual job at work requires the use of hearing protection, that would not be considered ‘frequently’. Likewise, a normal work task which requires the use of hearing protection but only occurs at long intervals such as once a month is also not considered ‘frequently’.