How long should I leave peroxide on my teeth?

It depends. I assume. You are asking about a peroxide based teeth whitening system. If it was purchased from a dentist, then you really must follow his recommendations, because it depends on your gum condition, teeth sensitivity, peroxide concentration, and many other factors. If you bought it over the counter, then just follow the instruction on the box. Please, feel free to contact me for more information.
Do not use regularly. I believe the best way to use this is in the tooth pastes containing it.
Depends. Are you using hyrdogen peroxide or carbamide peroxide? Is it a gel form made to whiten teeth or are you using hydrogen peroxide out of a bottle. Need more info to answer your question.