I assume 1-3 hops of modern switches without congestion. Given 100Gb/s lanes these numbers are possible if you get all the bottlenecks out of the way. The moment you hit a deep queue the latency explodes.

So, are you talking about theoretical latencies here based on bandwidths and cable lengths, or actual measured latencies end-to-end between hosts?

I know that "in principle" the physics of the cabling allows single digit microseconds, but I've never seen it anywhere near that low even with cross-over cables with zero switches in-path!