Under water breath holding?

I frequently have breath holding contests with friends in swimming pools and sometimes times myself. Usually i reach 30 seconds under water until it starts to hurt like hell. But i was watching magic stuff on youtube and came across David Blaine's breath hold and found out that when you hyperventilate until lightheadedness, you can significantly hold your breath longer but because you basically numb that sensation because of the amount of c02 in your blood (i reached 2 minutes comfortably). i know that doing this can cause brain damage and shallow water blackouts. How long would it take me (15, 125lbs, 5'5) to have brain damage set in and how long till a black out. i have a safeguard when doing this (lightly squeezing a friends hand and if i lose my grip they pull me out).