My plastic Jaz 8 gallon is at least 30 years old, as is the foam. The only problem is the filler cap sticks. If i can get it open, it still functions fine.
Modern tanks are the same plastic so they should last for ...oh 8-10 years until the rest of the car falls apart. Kidding, they should last a good long time. The plastic is impervious to the fuel but keep them out of direct sunlight. the UV is harder on the plastic than the fuel.