Natural pressure release or NPR is when the cooking time has ended and the valve is left closed so that the pressure inside decreases naturally. It takes time for the pressure to drop naturally, but the good thing is that it ensures your food is kept warm at all times. The release time also depends on the amount of liquid or the type of food inside the pot.
NPR can take about 5 to 30 minutes in an electric pressure cooker. When all the pressure is gone from the pot, the float valve drops and the lid unlocks and opens. However, there is no clear signal of when the pressure is completely released.