Dr. Bill Dorfman: There are actually two really popular gels that you can use for whitening your teeth. One is called night white you wear that at night it last about 5 or 6 hours once you put the gel in the other is called day white, and you do that during the day for about 30 minutes twice a day, now if you leave the gel in for 40 or 50 minutes is that can make your teeth whiter, no because what happens is that gel breaks down in hydrolyses, so after about 30 minutes the day time gel doesn't work anymore, and up to about 4 hours in night time gel doesn't work any more. If you like in your teeth whiter just do it again the next day.