how long does it take for your baby to self soothe?

Kayley • 24 years old, I have a little boy who’s 4 and I’m TTC my second baby!! 👶🏻
I've been going in and comforting my baby for at least 20 mins now, he's beyond shattered, but won't stop crying, as soon as I rub his head he goes to fall asleep but as soon as I leave the room he cries again, how long does it take for your baby to actually fall asleep using the no tears method??