If you've properly painted the plates (IE, surface prep to clean oils, sprayed primer, etc) then things should last you longer than anything else on the keyboard. The plates see very little touching or movement, as most plates are fairly well obscured by the keycaps, and often a top layer, so you don't have things rubbing the paint off.
Even if you just do a simple cleaning and spraying with rattlecan style spraypaint then you should have something that lasts a long time. Really all depends on the level of work that you want to put into it, and what materials you use.