The lasting effect of hypnosis really depends on individual.
Some people can enter into hypnosis rather easily. Some upon entering will stay in this state for quite Long. Some will drift in and out of the conscious and subconscious minds at will.
There is no hard rule that says you must stay in hypnosis for a certain duration.
In hypnotherapy, the hypnotherapist will use long induction method to get the person enters into a trance and stays there longer so that suggestions can be carried out.
Stage hypnosis, in contrast, uses short or rapid induction.