We get this question occasionally, and it is nearly impossible to answer with any accuracy. It totally depends on the user.
Things that can shorten the life of the silicone flanges are automatic dishwashers, lubricants, frequent sterilization, storing in refrigerator between cleanings, and frequency of actual use.
The best advice we can give to increase the longevity is to thoroughly clean the flanges between pumps using warm water and a good liquid dish soap, like Dawn. The silicone should be crystal clear when done cleaning and have that “sticky” silicone feel when dry. If it feels slick, there is still oil on the surface.
The hard plastic is less susceptible to the factors listed above, and generally last a bit longer, but again, it really is up to the individual user. We have moms that need to replace every few months, and others that use the same flanges for multiple babies.