Hi Benstark,
While there is no way to quantify how long any mattress will last for a specific person or predict exactly when you will decide to replace it because it is no longer suitable or comfortable for you (because this is the only real measure of durability or the useful life of a mattress that really matters) because there are too many unknowns and variables involved that are unique to each person … if a mattress is well inside a suitable comfort/support range and isn’t close to the edge of being too soft when it is new (see post #2 here) and meets the minimum quality/durability specs that are suggested in the guidelines here then it would be reasonable to expect a useful lifetime in the range of 7 - 10 years and with higher quality and more durable materials like latex or higher density memory foam or polyfoam (in the comfort layers especially) it would likely be in the higher end of the range or even longer.
There is also more detailed information about the many variables that can affect the durability and useful life of a mattress relative to different people in post #4 here and the posts it links to.
If you are in a more average weight range then the difference in the density of the polyfoam support core would be relatively minor because you would be compressing the deeper support core less than someone that was heavier but if you are in a higher weight range (more than the lower 200’s or so) that would be compressing the support core “through” the comfort layers more deeply then it could make a bigger difference and I would lean towards the higher density support core.
Phoenix