Have you seen this vid?
(I honestly find this quite a cute vid, (even without Linus' kids, as Linus is quite happy of the way he made his own keycaps, just like a kid would - or that's the way that I saw it).
This is die-sub, so in basic terms, although not exactly, the special ink is absorbed by the plastic, as opposed to something being stuck on. It also needs to be PBT.
In terms of how long the printing lasts, it should last very long. Let's say, longer than you personally would need the caps for. Some go as far as to say permanent. The ink gets sublimated into the plastic with a certain depth. After doing so, if you were to gauge/scratch the surface, you'd be able see the blank plastic underneath, and although it may appear to be a really thin layer, it isn't that bad. You'd have to wear out the plastic A LOT, but I'd consider that almost impossible with soft fingers (essentially scratching the top surface off a cap for that to happen).
Think of it like the yellowing of old ABS, or ideas on it. You can retro-brite the plastic to de-yellow it, but depending on how bad it is, the yellow may appear again over time due to the yellow underneath which didn't manage to react diffusing to the top layer (with the effect being much shallower than even 0.5mm for example). There still hasn't been concrete information on the process of plastics damage, however, there have been hypothesises that the above can be the case: classic-computers.org.nz/blog/2013-01-15-retr0bright-only-temporary.htm
What I am essentially trying to say is that the depth of sublimation may be more significant than may appear to the eyes.
Of course different materials and techniques can affect the keycaps quality/durability etc... I don't see this as hard at all, and definitely easier imo than resin caps. Hope this helped
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