Generally, ovens should be preheated to the specified Baking temperature. That is if the cake is baked at 190 deg. then the oven should be preheated for the same temperature. Also there will not be any specific time for this in my model. I will start the empty oven at the needed temperature and continue heating, when the oven reached the specific temperature, then it will beep indicating that the temperature is reached, then I proceed baking. Every oven is different, so please go through your owner's manual for exact way to preheat your oven.