If I do it myself (Which I decided to, to save the money), I use the Chemical Guys' line.
It's time consuming, and the Devil is in the details.. it all comes down to prep work.
For the ceramic coating, I use the Hydro Charge, which has a stated duration of one year.
However, in my experience.. that's one year if you do NOTHING else to it, and if you abuse it by running it through a tunnel wash instead of doing it yourself.
Here's what I do:
1) Wash the car myself. I use Adam's MegaFoam in a foam cannon connected to a Ryobi electric pressure washer.
2) Rinse the car
3) Clay Bar the car. I don't notice a difference between actual clay and synthetic, myself.
4) Depending on the condition of the paint, I'll either use a light cutting compound to get out the swirls and scratches, or not, depending.
5) I work in 2 foot by 2 foot sections, one panel at a time:
a) Wipe down the panel with 70% isopropyl alcohol to get any residual polish or soap off.
b) I use Chemical Guys' Hydro Charge, put on an applicator pad. In a cross-hatch pattern I'll go over that 2 by 2 section until it 'flashes'. Flashing seems to be when the binding agent evaporates out and
leaves behind the ceramic coating. Then I use a microfiber towel to lightly buff out that one area, before moving on to the next 2 by 2 section.
c) I'll let the car sit for about an hour, after I've completed the entire car. Then, I'll do step 5b all over again, ensuring I get the entire car covered equally.
d) I'll clay-bar the front and rear windscreen, and apply Hydro Charge to them too, using the same methodology. 2 coats. Then I hardly ever need to use my windshield wipers. The wind (at appropriate speed) blows the rain off.
Now, how long it lasts really depends on how you take care of it, from here. If you ceramic coat it and then run it through a tunnel wash, it'll last maybe 6-12 months. That also goes for the professional stuff, too. Tunnel washes are not friendly to your car, or paint. Or ceramic coatings.
Here's what I do to maintain the coating:
1) Once a week, I wash the car myself, using the Adam's MegaFoam or some other pH neutral soap.
2) Rinse the car with the pressure washer
3) Grab my 5 gallon bucket and fill it with 2 or 3 ounces of ONR (Optimum No Rinse), and wash the car again with that.
4) I dry it using a leaf blower. Don't laugh.. it actually works REALLY well, on a coated car. And prevents you from scratching your paint by grinding any missed debris or dust into it with a towel.
5) Once it's dry, I grab either Hydro Speed or Xpel booster. I lightly mist one panel at a time, then buff it out with a microfiber towel.
Both the Hydro Speed and Xpel are a watered down version of the coating itself, so it kinda boosts the coating, and 'fills in' any areas that've weakened or started to fail. If you keep this up, there's really no reason why the coating won't last fundamentally forever. The more religious you are about it, the longer it lasts.. especially if you park out in the elements, and the car's constantly getting baked in the sun or rained on.
Me, I park in my garage, so I've had 'One year coatings' last pretty much 5 or 6, in the past.
I admit, I tend to go through cars faster than some people do their socks, so I really can't give you an upper limit.
But if you put in the effort and do it properly, you can make it last WAY longer than what they promise.
