When should fiberglass insulation be replaced?

don't replace it. fiberglass will compress a little over time and lose some of its insulative function but shouldn't have to be replaced because it is still working just not as efficiently as when new. Add more if your'e concerned about it. Nowadays 6 to 8" is minimal and 10 to 12" in extremely cold or hot climes. It'll pay for itself fairly quickly. It's a little like sex, a lot is way better than a little!!!