Annually at minimum to comply with MSA from what I remember, as a former trainee Environmental Scrutineer.
From the same role I'm also aware of how difficult it can be to get an accurate reading. However, from MSA perspective it is overall noise, not just exhaust. The instructions are to not carry out noise tests in a confined area.
(It's a few years since I did any of this so rules may have altered and my memory is not quite what it was
)
