How well do teeth whitening trays from the store work?

I want to get the teeth whitening trays from the store. Do they work? How long do they take to work?
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Teeth whitening is commonly accomplished by applying a bleaching agent to the teeth. The longer the bleaching agent is applied the better it works. Or at least this is how every teeth whitening system is marketed. If you are really looking for teeth whitening, normal toothpaste probably isn't enough just by itself. I suggest doing what this mom did, http://laurateethwhiteningblog.blogspot.com/
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