Hi, I can tell you from my experience that bleeding out isn't as easy as it seems. I tried that method last year and I didn't touch any artery or vein, even though I was cutting my wrists. This year I started self-harming and cut deep 'til you get to cut an artery or vein is really difficult.
You have to also have in mind that if you cut your tendons in one hand while slitting your wrist, you might not be able to cut the other one.
From what I know, the best way is forming a fist with your hand and bend it towards you, turning more visible the gap between the tendons and the rest of the muscles. Then, in the gap where the radial artery passes through, make a deep cut to reach the radial artery. Then repeat the same on the other one.
Is a very painful method, so I recommend you to take painkillers to numb the pain and also accelerate the death.
Idk how long will take to bleed out and die, but I have read that maybe about 2 hours or more. Maybe I'm wrong. I don't know.
You have to also take into account that cutting your wrists is the easier than cutting any other part of your body, that's why i'm only talking about that. But, just know that even though it's a really common method to CTB, it's effectiveness is barely guaranteed. It's very probable that you will survive to that if you don't do it well.