It's one of those 'how long is a piece of string' questions. But it should last until both partners are satisfied. SexualHealth.org describes PE as an inability to delay ejaculation to a point when it is mutually desirable for both partners . Some guys boast that they can last for hours, and they probably can. This is achieved literally by practice.
Boys learn to bring themselves to the brink of orgasm and then back off for a while.
By doing this, they learn to control it. It's thought PE stems from boys learning to masturbate to achieve orgasm really quickly. Most guys have suffered PE at some stage of their sexual development - usually in the early days of having sex, or if there's been long periods between encounters. There's the squeeze technique where you literally squeeze the head of the penis for 10 seconds to kill off the orgasm. We can also work with him by slowing down when he starts to go into 'orgasm mode'.