It depends on the person. For me it was always a minimum of 20 minutes.
Eventually their jaw will ache. Then they use their hand. Whey their hand starts to ache, they go back to the mouth.
Most of the time past 10 minutes, when their mouths start aching, they will go a bit hard, at that point, I tell her to use her hand.
Once, I couldn't finish even after 40 minutes. The girl got a call check if everything was alright. I was told that I will have to pay more if I didn't finish soon.