Whitening gel works by penetrating the enamel to break down stain molecules - it does not erode enamel. Done at the correct concentration and protocol in a clinic, enamel is not harmed; any sensitivity is usually mild and passes within a few days.
Risks usually come from unregulated or DIY products with uncontrolled strength, which can cause lasting sensitivity and gum irritation. That's why whitening should be supervised by a dentist, with gum protection and a check of your teeth beforehand.
Results typically last 1-3 years depending on your habits (coffee, tea and smoking stain faster). Your dentist will advise on maintenance and when a touch-up makes sense.