Depends. It really depends on your lifestyle and the health of your teeth. Do you eat a lot of carbohydrates? Do you drink soda? Do you have a history of tooth decay? If so, your teeth will benefit from Fluoride treatments every six months or more. If not, you might not need it regularly at all--especially if you have never had a cavity before.
Depends. If you're a kid, and your mouth is more acidic, yearly. If your an adult, and your mouth is more alkaline, less often...Unless 1. High caries (decay) index, 2. Roots of teeth exposed, 3. Gum recession, 4. Gum-line cavities, 5. Multiple fractures. 6. Other risk factors. Discuss need with your dentist.
Opinions vary. I do not use fluoride. Other dentists will choose to use it. Do your pen research on Fluoride and you will be able to decide which dental philosophy is for you.
Needs based. Each mouth is different and the recommendations should be based off of your dental history. If you are at high risk for cavities once every 3 months might be ideal. If you are low risk once every 6-12 months might be right for you. Topical flouride is one of the best ways to prevent cavities.