The Rav of my shul is also a teacher at a nearby school. He can’t answer questions during class! He does have hours, but if you leave him a message, he will call you back. Generally he is fast.
One time, I really needed a fast answer and we couldn’t get through to him. I called his house and cell phone. It was early morning (7 am). I waited for about an hour but then needed an answer so I stalked him 🙂 I went to his house and rang the bell and apologized profusely. Turned out he had been in shul between 7-7:40 and hadn’t checked his messages yet.
Overall, I think it depends on the size of the shul and his salary. If he is being paid as a full time Rabbi (meaning, a real living wage) whos sole purpose is to be available to the shul, then his hours should be much longer.
If he is giving a smaller wage and has a regular job, his hours should be shorter.