In my experience it seems to depend on the merchant (or probably the processor they use in between AMEX and the point of sale). I think the other thing that might also matter is whether it is subject to changes, like a restaurant might go back and add a tip so it might be pending for a few more days.

With that said, I have never seen a consistent time frame for when something is charged through to when it posts. Frequently if something is charged through in the last day or two of a bill cycle it will not post for a couple days and will miss the billing window for the current cycle. YMMV though.