I have only been beeped once that I can remember, I don't think it was an inappropriate time though, I thought I said gone, but I was definitely gone longer than expected due to toddler aggro.
I would think if it is urgent, and you need that specific PC for an interaction. But only if you have been interacting IC a bit, like emoting/speaking to them, and you don't get a response after a few minutes? If that criteria is met then beep them, otherwise just move on or wait for someone else to come along? I usually just move on. Sometimes people might be just disconnected, I once saw a PC standing in the same spot for two or three real life days every time I logged in. So beeping them would have done nothing.
I think it says to use the beep command pretty sparingly. Yupp:
Use this command very sparingly, if at all.
I have never 'beeped' anyone before.