I have facemounted prints dating back to 1985 and they still look great.
As with so much in print presentation there are a lot of exceptions. For instance I have mural sized silver prints mounted on Masonite from the 70's that still look pristine. I think the skill of the mounting person has a lot to do with it.
I have mentioned before on this forum where a friend of mine had a nightmarish situation where many of the prints in his 100% facemounted show started to bubble and/or delaminate under the hot spotlights in the gallery. It was his first attempt at facemounting and he made a systematic mistake in the process, which was cleaning the plexi the wrong way just before applying the prints.
There isn't much consensus yet about how stable the facemounting process is. I suspect Diasec facemounting is probably a little better than the more common transfer-adhesive facemount technique.
But as far as Type-C's go, if they're displayed at normal room light levels (especially under fluorescent light) they will fade at a rate that is significantly faster than current pigment inkjet processes, and I can think of some examples. But maybe that's because that the final wash for the Type-C process is almost always minimal (1.5 minutes if your lucky) in the continuous process machines that are usually used. For my collection of Type-C's, the ones that seem to be in the best shape are the earliest ones that I manually processed in mesh baskets, where the final wash step was rather extended. The later ones were made with a continuous process machine with a very brief wash step, and they are the most yellowed.