Hi Kmfl98,
Durability is very complex subject and is difficult to quantify because there are so many variables involved but you can read more about the many factors that can make a difference in mattress durability post #4 here and the posts it links to. As you can see … how long a mattress will last is not as much about the complete breakdown of a material but more about the ongoing softening and gradual breakdown which finally reaches a point where the comfort and support of a mattress no longer is suitable for the person sleeping on it. In other words … durability is relative and dependent on the person as much as the quality of the materials and the design of the mattress. Someone who chooses a mattress that is “on the edge” of being too soft for them for example may find that the mattress doesn’t even last beyond the initial break in period of the foam before the comfort and support is no longer suitable for them while for someone else that is more in the middle of the range where a mattress is suitable for them may find that same mattress would last for many years.
1.8 lb polyfoam is a higher quality and more durable foam than you would usually find in a mattress and in a suitable two sided construction (where the layers are not too thick) can last for a long time (if you flip the mattress regularly) but it still wouldn’t be as durable as a well constructed one sided latex mattress IMO which is a significantly more durable material … again depending on the softness of the latex which will also affect its durability.
When you are in the range of HR polyfoam (2.5 lbs and higher) then the durability begins to approach latex in a more “apples to apples” comparison although latex would still come out ahead.
The durability of a pillowtop also depends on the quality of the foam inside the pillowtop but in most cases because the pillowtop is one sided, has thicker layers (which increases the negative effect of foam softening) and because they typically use materials that are less durable (typically low or mid density polyfoam in the major brands) … they would be significantly less durable for most people than either a two sided mattress that used thinner layers of the same or even lower quality/density foam or a pillowtop that used latex.
I think it would be realistic to expect that a two sided construction would increase the lifespan of a mattress by 2/3 over a similar one sided construction and perhaps more (although it won’t be double). You can see an example in the PS: of post #2 here of a manufacturer that I highly respect that makes some two sided mattresses that use 1.5 lb polyfoam and his "reasonable expectation (and he has been in business for decades so this is not just empty words) is that it will last “on average” about 10 - 12 years. 1.8 lb polyfoam would be a little higher in the same construction and design but also more costly.
Hope this … and some of the great earlier comments … are helpful.
Phoenix