I've got boards with PBT (Realforce, for example) and boards with ABS (Northgate Omnikeys with caps that are somewhere north of 25 years old), and I find that the keycap is one of the less important parts to the total experience. At my office, I often go back and forth between a Realforce and an Omnikey -- the switch feel is very different, but I don't notice the difference in the caps.
None of my caps has faded legends. (The old Northgate caps have developed some shine, but they were double-shot so the printing hasn't faded.)
Wasd claims that the UV printing process they switched to a couple of years ago produces a very durable cap on which the legends should last a very long time. I can't verify that claim myself because I don't have a board full of wasd caps that I've used all day every day for a couple of years. But the few cases I've found on-line where someone reports their experience over time with those caps, they're not seeing signs of wear.
So I'm guessing they'll be pretty durable -- you're probably talking years, not months, before you notice any fading, if then. Meanwhile, for $50 they'll sell you a complete set customized the way you want it, and there aren't many vendors that will do that. If you follow Massdrop for a while, you might find a drop there that does what you want. An example is this drop, which offers what should be a very high quality set of ABS caps, including enough extra caps to cover a pretty wide range of needs. It's currently at $130, might get down to $120 if enough people sign on.
https://www.massdrop.com/buy/massdrop-x-jessica-gmk-plum-custom-keycap-set
