Thanks so much for asking, kenpongo! Generally, the oven should be able to reach 350 degrees Fahrenheit within 20 minutes when only one oven rack is in it. However, there are a few things that could affect this, including the following: higher temperature settings; a voltage less than 240V (Although the range may operate at lower voltage, it will increase the time it takes to preheat.); and opening the door. ^Jecca