At the most I get about 15 years before the foam inside my various cases has degraded to the point of being embarrassing.  This is for dark gray foam, flat or egg-crate, original or replaced when new.  Same-appearance foam in some cases at work are nearing the gooey stage after about 30 years, while others look new.  All are stored closed at normal room temperature.

I have one large Anvil case (not guns) with shiny black dense closed-cell foam that still looks new after over 15 years.

What long-term results are you seeing?

I have the high density polyethylene foam and it is going strong after many years. I agree that the polyurethane (stock) foam does not
hold up as well.

I just replace foam periodically. I get rolls of it from "US foam by mail" (or something like that).