The wiki should be able to answer most of your questions, but I'll do it anyway:
Basically there are two common ways for illuminated keys.
The first: Make a clear/milky keycap and paint it black, leave out the letter when painting it. This is the most common method and will eventually wear off. Depending on the company and plastics use this can happen in a very short time. There have been several reports of Logitech and Razer illuminated keys wearing off extremely quick. Example:
The same goes for most Laptop keyboards that use the same method.
Second: Basically the same base idea as mentioned before, but instead of painting the black part is being "dyed" into the key under heat. This means that the paint will soak into the plastic and stay there. This method has been patented by TG3 for their illuminated keyboards. Since Deck is a daughter company of TG3, they use this method too. Its known to last for pretty much forever. You are more likely to spill something over your keyboards* before the keys wear off.
As for normal, non-illuminated keys there are various different materials and labeling methods and they all vary. Double-shot keycaps and dye-sublimated keycaps (same as the TG3 method described above) will basically last forever. Lasting forever is exactly what the problem of these technologies was. Lasting longer than the keyboards they are being used on is not optimal in terms of production costs, which is why these methods are nowadays only found on vintage keyboards or special equipment.
The most common labeling technology nowadays is laser-engraving. This means that the letters on the keycap are being "burned out" with a laser and then optionally filled with "paint" on black keyboards. This will also last very long but is known to have a much lower contrast than double-shot molding or dye-sublimation.
For more details, please check the "all about keys" entry in
the wiki.
