It depends. If you use regularly then you will have a hard time getting erect. If you only use for the first time of maybe a few times into it then you will get hard and stay hard for hours. You won’t be able to cum. You can have sex for hours but you’ll feel numb in the region. Heroin makes your penis feel a bit numb and you can’t cum. But if you’re a regular user you’ll develop what they call "dope dick". Dope dick is when you can no longer get hard. Heroin effects your testosterone levels and reduces your body’s ability to become sexually stimulated. Yep it makes you not get aroused. This happens around being on heroin for about a month. But before that happens you can still get hard and you will be able to have sex for hours, literally.