This question cannot be answered with a simple answer for your expectations about the final results needs to be realistic. Long term results can never be expected, but brushing, flossing and visiting your dentist for periodic cleaning and check-ups can only render optimum results. Also results of teeth whitening can vary from person to person. It is also relevant to note that not all can be a suitable candidate for teeth whitening.
Tooth whitening is not good for your teeth when teeth restorations like veneers have been used, or when there is internal staining of the teeth or root canal procedures have been done. Whitening products like hydrogen peroxide, and carbamide peroxide cannot work well on them and these teeth could appear artificial. Internal tooth whitening may be required when there is internal staining.
Brown or grey hue teeth cannot be whitened, with only teeth with a yellow hue producing good results. Teeth whitening may not be useful and is not advisable for pregnant or nursing mothers and those that have hypersensitivity in teeth.