Tooth whitening is a simple procedure that can restore a youthful appearance to your smile. Whitening removes surface staining using hydrogen peroxide. However, there are some stains that are much more deeply integrated into tooth structure called "intrinsic" stains that whitening will not usually be able to change. Tetracycline staining is a good example of this. Depending on the severity, whitening solutions may lighten these teeth some, but usually, these will be in need of more permanent, less conservative treatment, like veneers.
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The amount that teeth lighten is individual to the patient--in other words, some patients' teeth will get whiter than others, and it's directly related to the original shade of the teeth, the cause of the staining or shade, habits of the patient and the amount of time the teeth are exposed to the whitening solution or process. So, while patient A may immediately whiten a few shades, patient B may need to whiten more before seeing their desired result.
Sensitivity is a common side effect of whitening, since the solution opens the "pores" of the teeth, making them more susceptible to temperature changes. Although some people may experience more sensitivity than others, it usually only lasts a few days, and there are excellent products such as topical fluoride, tooth sensitivity gels and pain relievers to manage it in the meantime.
Touch up may be required every few months depending on the habits of the patient.