When sleep training, how long should i let my 1 year old cry for?

My 1 year old still wakes up 3 times a night to nurse himself back to sleep, we've stopped the nurseing and are begining the sleep training. I let him cry, but its blood curdling screams. sometimes this goes on for over 20 minutes. Should i just let it go one for as long as he needs till he falls asleep?