I find Google's version of this pretty zippy and useful, so I'd expect it will be at least as good as that. It's usually a ~5 second turnaround, I'd estimate; I typically use it if I'm walking to catch a bus or suchlike and don't want to slow down or have an accident by typing at the same time.
I think it depends on the carrier as well. Sometimes on my Droid 1 on Verizon it can take a good 10 seconds to get a get a response or 10 seconds to make the initial connection to the server to allow me to start talking.
