Couple of answers
1 Approximately 2 years or more with proper washing and maintenance.
2 Yes, and it's encouraged! Apply Ceramic Boost every 4-6 months to keep the coating smooth, looking great, and working as intended.
3 You can wax on top of the coating, but it is not recommended. It will alter the water repelling effect to a degree. Stick with Ceramic Boost, but if you really want to put a wax over it, Patriot Wax would be your best bet since it also contains a small amount of SiO2 ceramic in it.
4 Yes. You can use Coating Prep on Paint, textured plastic, rubber trim, chrome, headlights, etc. Anywhere you plan to apply a ceramic coating, the Coating Prep is safe to use to clean that area first.