How long does it take to get used to a metal saxophone mouthpiece?

Try choking up less on the mouthpiece, like have as little of the mouthpiece in your mouth as possible, make sure you have your tongue on the reed before you try to make a sound so you can build up the proper air pressure first and then experiment with how tight your embouchure has to be on the mouthpiece. Also make sure you're breathing correctly, blow out as if you're trying to fog up the glass on a cold window, like hot air.

Expect it to take 6 months minimum. It doesn't always take that long, but your chops obviously aren't even close to developed enough to control that mouthpiece yet.

Though I do have to ask, what is the tip opening/mouthpiece you're playing on right now? A D8 is a huge step away from legit mouthpiece tip openings, so it's most likely you if that's the case. If you're coming from something that was fairly bright and pretty open(~.078 alto, .105 tenor), then the mouthpiece you got being a lemon is fairly likely.

Man there are cats that never get used to playing on a Dukoff. Tough pieces to play. Bob's right. If you're coming from a Selmer /Vandoren to a D8 it's a real monster. leap. Might be too far a one for now.

Maybe you'll get used to it shortly, maybe you never will. Obviously, you won't be playing it much sounding like that, so how could you ever get used to it. Get what's right for you, not what others tell you is right or in or cool. I don't care for the taste of metal in my mouth at any time, so maybe you'll never like it. could be you'll never get a decent note out of it since it puts you off trying.