Basic principle: Up to 180ml of food at room temperature will take around 3 minutes to reach the desired feeding temperature. The time required may vary based on, for example, a different starting food temperature. Food taken from the fridge will take longer to reach the right temperature, for example. More than 180ml of food will also need a little longer.
Guideline: 220ml of food at room temperature will need around 5 minutes. A potential cause of food heating up more slowly is that the heating element with temperature sensor system has limescale on it. This hinders the release of heat from the heating element, or prevents it from registering the correct temperature. For optimal use, it is therefore important to clean the element thoroughly after each feed, and to descale it regularly. Refer to the user manual supplied (pdf) on this.