i would estimate 5-7 minutes. I imagine you could take longer than that finding a suitably quiet place on level 3 if the terminal is busy. Once you find the tunnel it is about 3-4 minutes, and the tunnel is 2 levels below arrivals. You can even purchase your refreshments at the 7 in the basement. It is the cheapest place to shop in the terminal as a lot of the airport staff go down there for meal breaks etc.

Just to add. The easiest way down to the basement is get down to level 1 however you choose, once on level 1 then walk all the way to the eastern end of that floor where you will find the escalator heading to the basement level (B1) go down that, walk straight ahead and the tunnel will be on your right.