Will be very difficult to calculate since the event is not steady state. To start off with, you will create a vortex at the entrance to the pipe with will disturb the flow going into the pipe. Then, air bubbles will be rising and water falling within the pipe with periodic bursts of air through the vortex.
In water the volume of air halves for every 10m of depth. Additionally, a bubble rising through water experiences forces of fluid friction and turbulence. These create an opposing force proportional to velocity, so just as a body falling in the air reaches a terminal (maximum) velocity, so it is for a bubble when the frictional forces due to its upward velocity match the upthrust.
A rising air bubble in water reaches a velocity of approximately 2/3 * (square root(gR)) for a bubble of radius R. What this means to the sound is that whatever the bubble size it quickly reaches its final velocity. Now suppose a bunch of bubbles, some big, some small, were created deep in the water. The bigger ones will arrive at the surface first, followed by the smaller ones.
From the sea floor 800 meters down, it has been observed that it takes 20 minutes for methane bubbles to rise 300 meters. Based on that empirical data, one can guess approximately 60 minutes.
You might get a better answer in the diving forum.