How long must I practice singing to sing some heavy metal?

I'm a guitar player, I'm quite good at it but I would like to be a front man in a band, not just a lead guitarist.. .. I like to play mostly thrash and speed metal. I can't sing at all, and when I try to it makes me cough. My idols are James Hetfield (Metallica), Dave Mustane (megadeth) , Rob Halford (judas priest) and Bruce Dickinson (iron maiden). How long does it approximately take to reach their level?