Let's clear up some myths about what a ceramic coating does for your paint.
Scratches are a small micro cut in your clear coat. They're usually caused by dirt or debris, but you can also scratch a car with a brand new microfiber on a freshly clean surface. Coatings increase the hardness of your paint, but won't prevent scratches from happening. Your best case scenario is that these small scratches are actually in the coating itself, and not in your clear coat. Thus we can polish off the coating and reapply it without the damage being in your clear coat (unless it's a deep scratch). Rock chips are also not going to be stopped by a coating. To prevent rock chips, install paint protection film (PPF/Clear Bra). Water marks and bird poo are another big myth. A coating can and will still etch given the right conditions. A black car which gets hit in the middle of the day by sprinklers and is allowed to dry will get water marks. It's the duration of how long those spots go untreated which determines how much damage is done. If you catch it on the same day you're a lot more likely to be able to remove the spots than if you waited a week. Wipe off bird poo with a waterless wash immediately to prevent etching.
A lot of people think etching is a chemical process. Etching occurs when one section of the paint isn't expanding or contracting equal to the areas around it. Over time this causes the paint to etch.
Layering is probably the biggest misleading information out there. 1 layer + 1 layer does not equal 2 layers. It's more like 1+1=1.2. The purpose of layering a product is usually to ensure full coverage and the difference in performance between 1, 2, 4 and 10 layers is minimal. Layering 2 seperate products (like a thicker, more durable coating topped with a more hydrophobic coating) does yield extra benefits. But layering 2-4 of one product won't give you significantly better results than 1 layer.
The "self cleaning" aspect of coatings is boasted a lot. The car will stay cleaner longer, but things will still bond to your car's paint. You'll still want to carry out weekly or bi-weekly washes.
No coating lasts a lifetime. 5-7 years tops with proper maintenance.