Longevity of Teeth Whitening Treatment Results?

Longevity varies from person to person depending on many factors. Host factors would include diet, habits and age. If one drinks a lot of coffee and red wine, you should expect a shorter duration of whiteness. If one smokes, same thing. And teeth get yellow over time as one ages. The pulp of the tooth shrinks, leaving behind more dentin and a more yellow appearance.
Teeth whiten with gel differently based on frequency, duration and concentration. Whitening processes vary in longevity based on depth of effect. Some products are fairly superficial and don't last as long. Processes that are brief will not penetrate as deeply as processes that exposes the teeth to the gel for long periods of time
All whitening should be maintained over time, some people every 6 months, some people avery 3-4 years.

Teeth whitening maintenance needs to be done on a regular basis which usally is every six months.  Much like keeping a youthful face with facials, and a tan year a round by going to a tanning salon, teeth need to be whitened periodically to keep them white. If you practice good oral hygiene habits and brush with a whitening toothpaste and avoid foods that stain your teeth, maintenance will be easier.  Age also has a bearing on how white your teeth come up. Older people have a harder time than younger people . Unfortunately, age affects the whole body including your teeth! 

Teeth whiten to a different degree for each patient. That being said, each patient should expect the whitening process to last differently as well. Depending on what you eat, drink, and the habits you have, like smoking, determine how quickly your teeth will become stained again, but understand that there are ways to keep you teeth whiter longer after you have had them whitened.
I recommend that each of my patients whiten their teeth, top and bottom at the same time for an hour or two each month, with the take home trays that we have made for them. This does not stop the staining process altogether, but certainly helps to slow down the natural staining that will occur over time.