I don’t think there’s any ‘usual’ time. Comments are generally approved by the author, so it will depend on them. (I assume that admin *can* approve comments, but I don’t know if they do so, as a rule).
It *may* be that only first-time comments need to be approved, i.e. if you’ve had a comment approved before, you’re deemed ‘trustworthy’. So that *might* explain why other comments have appeared whilst yours hasn’t.
Sorry, that’s all a bit speculative. I’d maybe give it another day or two, and try commenting again. The author can always approve one and reject the other, to avoid duplication, if it comes to it.
