It depends what you mean by sex! The time people devote to doing sexual things can last from a few minutes, to several hours, or even a whole day! The actual act of sexual intercourse will often last until a man has an orgasm (ejaculates or comes), although there's nothing wrong with stopping before this point. A man might find he comes very quickly the first time he has sexual intercourse. Usually sexual intercourse lasts longer as people get more experienced and know what to expect. But with a new partner it can take time for people to get used to each other. Sex will usually be different every time - it depends on how a couple feel and what they want.