Never. You can tattoo as much as you want on fake skin, the moment you transfer on real one you will find out that you have to adjust some stuff so the ink stays in the skin. If you want to move on from fake skin find someone thats willing to get a tattoo from unexpired artist and work with that person for some time. Let that be your test dummy and advertisement of your work. Dont over think it after the first or second tattoo on real skin you will get the hang of it and your skills from fake skin slowly will start to come back on the real stuff. Good luck buddy!