If you prep it and leave it in the syringe it won't be big deal a couple hours later, but if you leave it overnight, a majority of the heroin will have turned into morphine. It'll still get you high, but it won't be heroin anymore or at least not for the most part.
Edit: Poster above me is completely right. Should definitely mention that it's not sterile by any means to do this. Although it may not end up being a problem for and most likely would not be, it is in your best interest to not leave the shot in the syringe for any extended period of time due to this allowing any bacteria already in the water will begin to grow exponentially in warm, stagnant water. The fridge idea would slow this down, but not prevent it.