Hi izzy,
There are many parts to what makes one mattress more durable than another for a specific person so this is a rather complex and relative question that can’t be answered in specific terms. It would depend on the specific design of the mattress as much as the materials themselves and also on how suitable your mattress choice was in terms of comfort and support (particularly support). For example if choose a mattress that is “on the edge” of being too soft for you then even the initial foam softening over the first few weeks could put you over the edge and the mattress may no longer be suitable for you even though the materials would not have worn out yet. In other words it would be “worn out” for you but not for someone else. If on the other hand you choose a mattress that has some “room” for softening and is still suitable for you after it goes through it’s initial softening and break in period (first 90 days or so) then further softening would be much slower and it would last you much longer.
There is more information about the many factors involved in durability in post #4 here (and the posts it links to) though that can help you decide whether one mattress or design is likely to be more durable than another. The most durable foam material would be latex and if you are looking for an innerspring then I would choose one that had low gauge (coils 12.5 - 13) and where the springs were connected together with helicals (not pocket coils) and were made to hold higher weights … and of course used high quality foam on top of the springs as well. Polyfoam in the range or 2.0 lbs or better, memory foam in the range of 5 lbs or better, and latex are all durable materials. Firmer versions of polyfoam and latex will be more durable than softer versions … especially in the upper layers.
Phoenix