Teeth whitening is generally safe when done correctly but has some risks like temporary tooth sensitivity or gum irritation. The effects typically last 6 months to 2 years depending on lifestyle factors like diet and oral hygiene habits. In-office whitening provides the fastest results, often lightening teeth by 2-8 shades in a single appointment lasting 30-90 minutes, but the results will not be permanent and touch-ups may be needed to maintain the whiteness.