It's 160 bytes for a standard SMS, plus any headers and other padding. Say 200 bytes in total.

Still, at 9600bps (standard GSM data rate IIRC) that makes the time taken to send the text (again, ignoring any handshaking and so on) 1/6th of a second. And given that 9600bps is a tiny fraction of what the network- even what individual masts- can take, it really is pretty pathetic that we're charged so much for texts.