There are different types. The ones that last longer and give more protection are sold only to installers as if you ess up the installation, is difficult to repair it and is much harder to apply
The "consumer" ones are easier to apply but usually last 2-3 years. At least in cars.
Bikes is a bit different as they are usually not exposed to as many elements as cars so a consumer product like opti gloss would work fine and likely last for a long time, especially if you just ride on roads and don't leave the bike outside.
Before applying, you MUST remove all oils, wax, etc. You need a paint cleaner for that. Is not enough to wash with regular bike soap. The coat will not bond to the pain if you don't prep it correctly.
In glossy paints, especially older ones, would recommend clay and potentially buff scratches off before coating it. Matte pain, you can't polish. So just try to decontaminate the pain as much as you can and make sure you use a matte coat. Thats why is ideal to do it when the bike is new.