There is no simple way to measure your accumulated sound level exposure if you use IEMs.
Sound Exposure Dosimeters work with built-in microphones and are placed on your body, so they do not work with headphones of any kind at all.
Even if dummy ears were used to get a reference, there is no standard because all standards are related to free air sound pressure level, so don’t expect any figures you can rely on.
18 hours a day is a very long period, so you need to stay way below the standard level for workplaces.
As a rule of thumb you should not go over the level of a normal human, talking to you.
This is relatively quiet, but one can get used to it.
In general, for such long periods, you should go as quiet as possible.
I suggest to use ADI-2’s loudness function, the boosted bass and upper treble has a lot lower payload on your hearing and makes it possible to further reduce the listening level, while keeping a natural sound balance impression.
Sidenote:
Wearing IEMs for such a long period per day might be physically unhealthy because of the lack of air going through your ear channels.
Long-term you might suffer from mycosis and other infections.
Think about mixing the listening time with circumaural headphones.