“How long does it take to whiten teeth with Crest strips?”

It is impossible to answer this question, as I do not know what shade your teeth are currently. Teeth are not white, they fall into a yellow hue, gray hue, or combination of yellow and gray. Has any extrinsic staining been removed by a dental professional? See a dentist, they likely can bleach your teeth and get a stable result compared to Crest White Strips.