Great question! Obviously, individual results will vary based on your particular bike, setup, and body, but most people can expect to ride 1-1.5 mph (1.5-2.5 kph) faster at the same power output when you ride in the aero position (vs. riding with your hands on the hoods). 

That translates to roughly 5 minutes saved during an Olympic-distance triathlon, 11 minutes in a half-Iron distance, and 22 minutes in a full Ironman!