How long will this memory foam mattress last before it sags?

It scores 9.1 for durability and carries a 10-year warranty. The comfort foam is genuine high-density foam, which resists the body impressions that retire low-density plush beds early, and it sits over a high-density Serta Foam Core with no coils to fatigue. That construction is what keeps the deep-cradle feel intact, so the plush surface should hold its character well into the next decade rather than collapsing within a season.