Hi thecountzero,
That depends on the quality/density of the polyfoam, the thickness, and where it is in the mattress. There is some very good quality and very durable polyfoam and some that is very a very low quality and cheap material. Post #4 here and point #4 in this article include the guidelines I would use.
There is also more detailed information about all the factors that can affect the durability and useful life of a mattress relative to each person in post #4 here and the posts it links to.
Phoenix