Your teeth will not whiten past your natural color (the color of teeth you are born with) - this is a common misconception about teeth whitening. If you're hoping for "paper white" teeth like you see the celebrities sporting in magazines, then you should check your expectations, most people are not capable of lightening to this shade. It's completely natural for your teeth to have yellowish hue or be an off-white color. The smiles in those celebrity photos are often enhanced with veneers, powerful photography lights or brightened in post-production with Photoshop! There are some people capable of whitening to a "paper white" color, because that is their natural shade, but if you were not born with paper white teeth to begin with, then you will not be able to whiten to this shade. Power whitening does not alter the color of your teeth, it only lifts the existing stains on your teeth.
So if I can't whiten past my natural shade, what IS my natural shade? That is something that can't be determined until trying multiple whitening sessions and seeing the lightest shade your teeth can brighten to.
For example, say you do three 60 min whitening sessions throughout the year. The first session you lighten to Shade 6, the second session you lighten to Shade 4 and the third session you lighten to Shade 4 again. It could be that your natural shade is a Shade 4 and you can't whiten past that.
Just like your natural shade, your whitening results are dependent on your genetics. The porosity of your enamel and the yellowness of your dentin underneath the enamel play a huge factor in how your teeth lighten and how bright they appear to be.