Set the expectation before you have to defend it.

Unstated response times get filled in by the most anxious assumption.

If you never say when you reply, people invent a standard, and the invented standard is usually faster than anything sustainable. Then a perfectly reasonable two day turnaround reads as being ignored, and the second message arrives with an edge to it. That edge is not unreasonable from where they sit. Nobody told them, and silence in a medium that can be instantaneous feels like a choice.

Stating it plainly, in a place families will encounter early, converts a source of friction into a known fact. Two working days, or whatever is honest for your load, is entirely acceptable when it is announced. It should be paired with a clear alternative route for anything genuinely urgent, because the reason people escalate is usually that they do not know what else to do. Give them the other door and they will use it.

Then hold it, including in the direction of not being faster. Replying at eleven at night, even once, teaches everyone that you are available at eleven at night, and it quietly sets the standard for colleagues who cannot match it. If you work at odd hours, and many educators do, consider whether the sending needs to happen then too. The message written late can leave in the morning, and that small gap protects a boundary you will otherwise spend years defending.