Basal body thermometer reading?

My fiance and I are TTC, and today I bought a digital basal body thermometer. The instructions didn't really tell me how long to leave it in my mouth. It said "Estimated time: 60 seconds." But my temp was still rising at that time. It doesn't beep when it's time to take it out. Should I wait until the temp steadies, or should I go for 60 seconds?