Not much. 2-5 roughly, depending on atmospheric conditions and such.
A friend of a friend of mine has an old LeBaron conv't and he hade his own CAI
with a 6in wide heating duct hooked up to his airbox, with a metal funnel hanging under his car at the other end. He said, "Yeah, it may look wierd, but you can't really see it, and it adds like 10 hp at least."