To boil a mixed rice porridge, it typically takes around 20 to 30 minutes. This duration is based on the assumption that the rice and other ingredients are pre-cooked or have been soaked for an adequate amount of time. The boiling time can vary depending on the specific ingredients used, the consistency desired, and the size of the porridge pot.

The reason for this time frame lies in the cooking process of the rice and other grains. When boiling, the heat helps to soften the grains and release their starches, which contribute to the thickening of the porridge. For instance, if the rice has been soaked for several hours or overnight, it will cook faster due to the pre-softening of the grains. Additionally, the type of rice used can affect the cooking time; for example, short-grain rice may cook faster than long-grain rice.

According to a study published in the Journal of Food Science, the cooking time for rice can vary significantly based on the grain type and the soaking time. Short-grain rice, which is commonly used in porridge, can take as little as 10 minutes to cook after soaking, whereas long-grain rice may require up to 20 minutes without soaking. This data supports the general guideline of 20 to 30 minutes for boiling a mixed rice porridge, considering the potential variations in ingredients and preparation methods.