A Canadian company that makes these 'butter dishing' products describes their fiberglass grave liners as having a 100-year warranty.
The site description states that their Fiberglass liner
Non-porous – water resistant
Excellent cold weather performance
Prevents burial site from sinking
Independently tested to withstand 18,000 pounds top load pressure Computer designed and engineered
Vault base available for a 2-piece sealed unit
Lightweight and stack-able, easily installed, and have superior strength."