"Instant messaging" doesn't have to be "instant". You can just close the program and open it every other hour, just like with email (which also trigger notifications). Or better yet: don't even install the chat app. Just use it in the browser and open as needed.
The expectation that you always have to be online is also unreasonable. But if you're still required to put out a show for an unreasonable employer, you can always set it to "Busy" and disable notifications. This also signals to people that you won't reply immediately.
Of course some co-workers will want to "chat" when you answer them, but this is where you must set limits and say you'll only have time for chat outside your busy hours. This is normally perfectly acceptable IME.
The expectation that you always have to be online is also unreasonable. But if you're still required to put out a show for an unreasonable employer, you can always set it to "Busy" and disable notifications. This also signals to people that you won't reply immediately.
Of course some co-workers will want to "chat" when you answer them, but this is where you must set limits and say you'll only have time for chat outside your busy hours. This is normally perfectly acceptable IME.