There are houses built with foam blocks as the form for holding concrete (ICF) built in the 1970s that are still standing.

Make sure the foam used has the proper PSI rating, and make sure you have proper drainage to reduce hydrostatic pressure, and there's no reason these things can't last 'forever'. It's not like the foam is biodegradable after all.

Not biodegradable, no, but it does have this pesky vulnerability to gasoline and other solvents, which turn it into napalm. Thanks to the extremely low density, a little bit of liquid can dissolve a whole bunch of foam. So long as it's well-sealed, there's no problem, but I'm a little nervous when I see it being used to construct highway onramps and the like.