how do i know how long takes me to cum?

Come on man, nobody can tell you that! Do you masturbate? I suppose you do if you are asking such question. You should feel it for yourself. Do you know when you are going to cum when you masturbate? It is the same when you are making love to a girl. Anyway, if you can’t figure out now, you probably will after one or two intercourses. “ Practise makes perfect!!!!” Use condoms until you learn to control your ejaculation, so you wouldn’t get the girl pregnant and to avoid so many possible diseases. If you are not acquainted with the sexually transmitted disease, feel free to ask!