Personally, while I shoot slide primarily, I do not mount them as I do not project them. I look at the slides through a lightbox, simple space saving.
Basically like what I had said, if you do not project them frequently, maybe after a holiday you share with your friends and family members just a few times, there will not be much problem. Probably they will fade after 20-30 years? If you really want to keep them well, you got to keep them in the fridge or even freezer, which is low humidity (30%), low temperture (10degree in fridge and -10 degrees in freezer, both approximated) and away from direct light (UV etc harmful light) for prolonged period. The humidity and temperture in fridge I just measured yesterday, think pretty standard or around there. Fuji called for a <25 degree for 10-20 storage of processed slides, <10degree for more than 20 years. Fridge is the way cheaply.
I will write up an article once I tested it to help others to store the precious slides/negative.