When that happens to me, I'll give it a reasonable amount of time based on the importance of the question (which could be a few hours or a few days) and then gently prompt with "hey, sorry to bug you, wondering if you had a chance to look at my question?". That has never not worked out for me.

If it's such an emergency that it can't wait, then I can call them. But most of the things we think are emergencies aren't, and most things can wait longer than we think.